Engine size when using twins

Just curious why most twin engine displacement trawlers have engines that are the same hp as would be used if the boat had a single screw?

For example my 36 fd trawler has two 62 hp engines. Since 62 hp is more than enough power to cruise at hull speed wouldn't it be more efficient to have two 31 hp engines and still be able to cruise at hull speed?

For production boats it makes manufacturing less complex and less expensive and thus the boats can be sold for a more competitive price. Also for boats with semi-planing hulls like Grand Banks, the operator can use the extra power to go faster. For example the final version of the GB36 could be fitted with one or two Cummins 220 hp diesels. The twin version can be cruised at speeds of 12 knots or more if the operator is willing to pay the fuel bill.

What Marin said, plus when your boat was built, there were very few choices for small diesels with matching transmissions. If one were building a boat now, the number of manufacturers and the range of HPs is amazing.

Ted

OCD if you wanted a GB36 for cruising 7 knots GB could have offered two engines w/o even needing to offer a different brand. But there's more to it ... the hull isn't even suitable for a speed of 7 knots. So if the hull was given an up turn to the stern that would have made the 36 a FD boat then 7 knots would be perfect and require 1/3 as much fuel. Would have been a lighter boat that definitely would have been a benifit also. But then 160hp (twin FL 80s) would be too much power so smaller engines would be in order .. such as the small Perkins. And there were plenty of Perkins engines at that time. But GB didn't offer any of these variations that would have been close to ideal. The GB36 had only one configuration that made sense and that was a twin FL 120hp at 8 to 10 knots.

There was a few years ago a GB 36 on Yacht World that had been repowered w two 55hp Yanmars. Was probably perfect for 7.5 knot cruising. GB could have done it years ago w Perkins engines ....... but they didn't. Their bad IMO.

manyboats said:

GB could have done it years ago w Perkins engines ....... but they didn't. Their bad IMO.

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They didn't do it because their market wouldn't have touched it with a ten foot pole. Their market was not and is not people like yourself who are willing to creep along in a displacement boat with a tiny bit of horsepower. The fact that they have sold thousands of boats worldwide bears that out. And as time went by, their market wanted even more power so they would have the ability to cruise slow and economical when they had the time and go (relatively) fast when they were in a hurry.

Their market for their new boats was and is relatively wealthy people to whom fuel cost is not as important as other considerations. American Marine was very smart in their understanding of their market. Had they followed your line of reasoning they most likely would have gone out of business long ago.

Totally agree with Marin. The market for those boats called for more power and as engines were developed with greater output the market scooped them up. Fuel costs may hit the owners further down the line but apparently not when purchasing a new boat. As pointed out the boats can travel slower with less fuel burn and the fact that a FD boat could do better hardly enters the picture the dye is cast at that point. So when fuel prices go up you see a lot of boats not designed to travel at 6-7K putting along, but when fuel is less expensive the same boats may be doing 10+ K with steep wakes.

manyboats said:

... The GB36 had only one configuration that made sense and that was a twin FL 120hp at 8 to 10 knots.

There was a few years ago a GB 36 on Yacht World that had been repowered w two 55hp Yanmars. Was probably perfect for 7.5 knot cruising. GB could have done it years ago w Perkins engines ....... but they didn't. Their bad IMO.

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Those GB 36 numbers sound very close to my cruise/WOT numbers with Perkins 85's. I think their approach to using larger engines makes sense and attracts more buyers. Most 34 Californians have the larger engines from 185s up to as high as twin 250s.

Not many powerboaters would accept a cruise at close to 6 kts. That's what sailboats are for.

Marin you're notion that everybody wants to plow along as fast as you is more correct than wrong but misses the mark by a mile thinking everyone's like you. But the market shows you're more right than wrong.

Al, your comparing a hull more like a planing hull to a SD hull that falls about in the middle between a planing hull and a FD hull. Not apples to apples. So yes .. the Californian needs more power per ton of displacement.
Sailboatx go slow because they designed for the power they have .... very little. And most power boaters have a problem w 6 knots because of their automotive heritage.

And the automotive notion that you can't have too much power enters into this a lot.

And yes a huge part of this is that the boats are bought new and those owners nearly always have plenty of money for buying fuel. And later on the boats wind up in the hands of the poor ... so to speak.

But w a FD boat you need "X" amount of power. Any more and it's a waste of power or you're forced to run forever underloaded or overdrive your boat.

But my post #4 is still 100% true.

Interestingly if I was buying a GBS 36 I wouldn't even look at a twin .... though I prefer twins. I would want to run it 7 knots and I'm not a fan of under loading engines. I would wish it had twin 40hp engines like I've got in Willy and the stern modified into a FD shape. Getting out the sawsall and modifying the stern would be fun ..... but a big job for this old guy.

manyboats said:

But my post #4 is still 100% true.

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Only for you. Not for the thousands of very satisfied GB owners all over the world or for the thousands of American Marine and Grand Banks employees over the years who had jobs because their company made the right decisions regarding hull type and engine power.

Kudos to those blessed with the patience to run motor boats at 6K or less. Many sailboats don't have a choice and many sailors become power boaters because they wanted to go faster than their previous sail boats did.

When you have no specific place to be from day to day....a trip of 5 to 10 miles at 6 knots isn't all that bad.....

manyboats said:

There was a few years ago a GB 36 on Yacht World that had been repowered w two 55hp Yanmars. Was probably perfect for 7.5 knot cruising. GB could have done it years ago w Perkins engines ....... but they didn't. Their bad IMO.

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Al's comparing a hull more like a planing hull to a SD hull that falls about in the middle between a planing hull and a FD hull. Not apples to apples.

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The point is folks like many GB owners enjoy going 12-15 kts. The 34 Californian LRC can go 23-25 kts with the right power...more planing than SD.

The more speed options you have, the greater appeal to the masses. I'm sure the folks who built these boats for a living are pleased with the business decisions made.

Sure Al ... my favorite speed to run is about 12 knots. But for the huge amount of fuel it would take I'd love to do it. But we can go 1000 miles and more at 6 knots ... as retired boaters. I don't think speed is sinful.

I don't like seeing SD boats being powered w only half as much power as they need. Or (put another way) w an engine ideal for FD speeds but w a SD hull. I'm calling it bad design ... HAHA but as I said if I was buying a GB36 I'd get the single. Perhaps I'd make it better and perhaps not.

And I should add that GB isn't the only boat w this problem. Many others are designed the same way.

I don't see it as a problem and neither no thousands of GB buyers like this guy.

manyboats said:

And I should add that GB isn't the only boat w this problem. Many others are designed the same way.

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You need to take a sales and marketing class, Eric.

It's not a "problem" other than in your eyes. It's intelligent market analysis and then designing and manufacturing a product to effectively satisfy that market.

The boat you keep envisioning wouldn't sell except to a market that is so small that the only way to make a profit would be to price it high enough to be profitable with a very low volume of sales (can you say "Nordhavn"?)

A production boat like Grand Banks and the other makes aiming at the same market do what they do (or did what they did) to make their product appeal to a large enough market to warrant putting the boats in production with a reasonable profit to the manufacturer. And the more boats of the same type you can produce, the lower the price to the consumer can be, and the bigger your market gets, so the more boats you can produce and the better your overall profit will become, and the upward spiral continues (they hope).

But--- this upward spiral only happens when you have a product that meets the requirements and desires of a market that keeps growing. The semi-planing boats like Grand Banks, Californian, CHB, Island Gypsy, Bayliner, Sabre, Eastbay, Nordic Tug, American Tug, etc do. A slow, low powered displacement power cruiser does not.

And upward spirals are what companies want, be it Grand Banks, Boeing, Apple, Tesla, or General Electric. (Tesla has announced a lower-priced version of their car to help them capture a larger chunk of the electric car market.)

Regardless of whether or not the boats Grand Banks and the rest produce/produced have any appeal to you, or meet what you feel defines an ideal cruising boat, the fact they were produced and sold in large numbers (many thousands in the case of Grand Banks and probably similar numbers with the other popular brands) proves their market analysis and subsequent product designs were the right way to go.

So you are sort of arguing with yourself since Grand Banks and their competitors and the buyers of all these boats aren't interested in what you're trying to "sell." The manufacturers are too smart to make it and the buyers aren't interested in buying it.

The only way you are correct is if market success is, in fact, a problem.

Still a boat should have the appropriate amount of power for it's hull design and weight.
Can you find any flaw in that?

Some guy put an 80hp Cummins in his Willard 30. And it made 8 knots at WOT.
That same engine in a GB36 probably wouldn't give it hull speed.

Both examples are of bad design. There is only the smallest of ranges of power for a FD boat and a small range of power even for a SD boat. Doubling or halving the power will always produce what I call a bad design ... or it was a bad design to begin with. A good design will have an appropriate amount of power.

manyboats said:

Still a boat should have the appropriate amount of power for it's hull design and weight.
Can you find any flaw in that?

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Absolutely. If the boat won't sell, then it's a bad design. It's that simple.

Other than the handful of people who like to build boats for themselves and so build what they think the boat should be, building boats is all about selling boats. If you can't sell them, there is absolutely no point in building them. Which means a design that won't sell--- no matter how efficient it might be in terms of power to hull shape--- is 100% worthless.

Here's a car analogy, sort of. In the 1950s, they began putting fins on Cadillacs. Did the fins improve the car's efficiency? Nope. Did they improve the acceleration? Nope. Did they improve the car's handling? Nope. Did they improve the car's safety? Nope. Did they do ANYTHING to make it a better automobile? Nope.

But the market went bonkers. Fins were the coolest things since sliced bread. So for a bunch of years the car makers put fins on everything. Good design from the point of view of making a better, more efficient, better handling, safer car? God no.

Good design from the point of view of selling lots and lots of cars to a huge and getting huger market and making lots and lots of money? No. It was a FABULOUS design.

So that's the way it is with boats. Your low powered, displacement boat is a car without fins in the 1950s and 60s. It's a bad design because nobody wants it.

There's theory, which is where you're coming from, and there's reality, which is where Grand Banks and Californian and SeaRay and CHB and Bayliner are, or were, coming from. And reality trumps theory every time.

In your theoretical world, a boat should have the appropriate amount of power for its hull design and weight. No quarrel there. But in the real world, people want a boat that can go slow when they want to, and go fast(ish) when they want to. Look at the photo of the GB that FlyWright put up a few posts ago. That's what that boat buyer wanted to be able to do. And water skier or not, that's what the bulk of the cruising power boat crowd wants to be able to do.

I have never met a power boater who didn't wish his or her boat could go faster. Not necessarily all the time, although there are plenty of people (like me) who would like that. But at least to be able to go faster some of the time.

Do you think it's right to use a cruising boat to to pull a water skier? Yes, no? Doesn't matter. The guy who bought the boat wanted to use it that way, and Grand Banks made a sale if we assume the guy driving the boat is the original owner. So the Grand Banks design is perfect for the thousands of people who bought Grand Banks boats. Which makes Grand Banks' formula of power to hull design and weight right on the money. Which makes it... a great design.

I kinda like the fact that my Lehman 120hp is, by Eric's standards, over -powered, but it's nice knowing if I had to beat into some nasty stuff, it has the power to do so with ease, and still 'not be breathing too hard'.
In fact, when I'm going anywhere, even at hull speed it's not breathing hard - nowhere near WOT, but always at normal op temp, which is surely the main point. But it has done 5000 plus hours, (I say plus because the hr meter died some time back, so it's a guess), and it uses virtually no oil, and runs like the proverbial sewing machine.

I often think the issue of over-powering and under-loading is done to death, anyway. When I go out in my old 1990 GT4, it has the potential for 220kph, but I (nowadays 'cause I'm respectable), never, well, not intentionally, exceed our limit of 100 or 110 kph. I'm sure the engine just loves that, as it still uses virtually no oil, and it has clocked up 300,000 mms.

I too also like the reserve HP for those few times you need it.

We were crossing the straits of Georgia NewCastle Island to Welcome Passage in BC, running with a 42' single engine TT who only made about 6 knots.

Normally I wouldn't care about running slow, but the weather came up and we were getting our behinds kicked wallowing around in the slop with 20 miles of ugly highway ahead of us and three kids who were turning green. We started the second engine, wished the other skipper well, kicked it up to 11 knots and made the crossing in half the time.

Very thankful to have the extra speed on occasions like that!!

Our Tolly can easily cruise at 16/17 knots and hit 21 plus if desired. She can also lope along at economical hull speed or below. All that is just what I want her to be able to do!

One of the many reasons we purchased.

Different strokes fer different folks!!

Marin I'm talking about a good boat design. You think it has something to do w sales and you're right. But because people by a boat dosn't make it a good design. People buy all kinds of bad products. If we knew everything about all the stuff we buy there would probably be a lot we'd Huck out.

You guys just think you need all that power. Why is it that you all need that power and I get along gracefully without it.

manyboats said:

You guys just think you need all that power.

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No you're just looking for an argument Eric.

manyboats said:

Why is it that you all need that power and I get along gracefully without it.

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Because I enjoy traveling faster than you do? Because there's perhaps 1 full displacement power boat for every 500 semi-displacement boats on the water?

Apples and oranges Eric, comparing a Grand Banks and a Willard hull is akin to comparing a Camry to a T-100 pickup.

PS, it has everything to do with sales because nobody builds used boats. They build new boats to sell to folks with the disposable income to buy them.

All that matters is that you like your boat. My 80-horsepower engine is nicely balanced pushing the full-displacement Coot's 14 tons economically a knot under hull speed at 1800 RPM and reaching hull speed at 2200 while the engine has a maximum rating of 2400 RPM. See no advantage of over-propping or having additional horsepower in my situation.

Peter B said:

When I go out in my old 1990 GT4, it has the potential for 220kph, but I (nowadays 'cause I'm respectable), never, well, not intentionally, exceed our limit of 100 or 110 kph. I'm sure the engine just loves that, as it still uses virtually no oil, and it has clocked up 300,000 mms.

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I just finished a shoot in Taiwan at which one of Boeing's top service engineers was a consulting expert to the crew doing the work we were shooting. Thia guy, in addition to what he does for Boeing, lives and breathes performance cars and motorcycles. Stock cars, dragsters, motocross bikes, that sort of thing. He's not just an enthusiast, he builds or rebuilds them for a hobby.

At one point in the conversation one evening the subject of engine life came up. He made a statement which reinforced what I've heard from other very experienced mechanics and operators in the automotive, aviation and marine worlds. And that is that if you want an engine to last a long, long time, take it easy on it. The easier you make life for an engine, the less wear it will experience and the longer it will last.

I asked him what about the notion that engines need to be run hard, that that's what's good for them, they need that. His answer was one word: "bullsh*t."

He went on to say why in detail, but his basic premise was if you want an engine to last a long time, don't run it any harder than you have to.

manyboats said:

You guys just think you need all that power. Why is it that you all need that power and I get along gracefully without it.

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Because for whatever reason you're satisfied with cruising your boat at the speed a turtle crawls and we're not. Slow sucks. That's basically what it boils down to. Our boat with the power it has only goes eight knots at the rpm we use, which is the best rpm for maximizing the life of the engines. But even eight knots is a hell of a lot better than six to our way of thinking. If our boat had been built with much more modern and more powerful engines we would run it at twelve knots. That would be way better than eight.

We're involved with a boat now that cruises at 30-35 knots. And I'm already beginning to wish we'd put more powerful engines in it.

manyboats said:

But because people by a boat dosn't make it a good design.

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From whose point of view? Yours? You're living in a theoretical world where reality is of no importance. Or the point of view of the boat manufacturer, who's living in the real world of costs and payroll and taxes and profits? To him, your idea of a good design is his idea of a really crappy design.

People buy all kinds of bad products.

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Is a Fleming a bad boat? Is a Grand Banks a bad boat? Is an Eastbay a bad boat? Is a Krogen Express a bad boat? Is a Sabre a bad boat?

Theory doesn't mean squat unless it has a practical application. And in the world of production boats and the people who buy them, your low-horsepower, slow displacement boat concept is a non-starter because a) nobody but a handful of people want them and b) manufacturers are smart enough not to fool around making them if they want to capture as large a market share as they can.

Yesterday reached a speed of 9.5 knots (with the boat at normal knot-below-hull-speed power) thanks to a three-knot favorable tidal current. Not a rare occurrence. (Reminds me of sailboat racing days fifty years ago where "playing" the currents was a major tactical concern.)

Guys - - > Eric and Mark especially... but... for sure, due to words that have passed, Eric considerably more than Mark.

I've read every post in this thread. Bottom line as I see it, is that, I don’t really hear others telling you that you should have boats with more power… for some reason such as, “because you really don’t understand”…

Soooo, I recommend… Let IT Go! We all should simply enjoy the boats that please us at whatever speed(s) applicable.

Happy Boating Daze! - Art

Art said:

Bottom line as I see it, is that, I don’t really hear others telling you that you should have boats with more power....We all should simply enjoy the boats that please us at whatever speed(s) applicable.

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Absolutely. Nothing wrong or bad about a boat that's low powered, slow, and economical if that is what a boater wants from a boat. Just as there is nothing wrong or bad with a boat with a pair of 500 hp diesels that cruises at 20 knots if that is what a boater wants from a boat. The high-powered, semi-planing boat design is bad if what a boater like Eric or Mark wants is a slow, economical, displacement boat. The slow, low powered displacement design is bad if a what a boater like me or Art or Peter wants is a cruising boat capable of faster speeds when needed or desired.

The notion that has been put forward in this discussion that a cruising boat design is automatically "bad" or "wrong" if it has more power than is necessary to creep the boat along at displacement or lower speeds and that its manufacturer has made a bad boat is bollocks as they say in the more civilized parts of the world. Every boat design, and every power to weight to hull design formula is right on the money if the resulting boat delivers what the buyer wants out of a boat.

The design and power of Art's Tollycraft is absolutely correct for the kind of boat Art wants. The design and power of our Grand Banks is not quite correct for the kind of boat we want because we wish it had even more powerful engines so we could cruise the hull faster than eight knots without making life harder on the engines. And the design and power of Mark's Coot is absolutely correct for the kind of boat Mark wants.

So I'm thinking it's time to put a bullet in the head of this notion that the only boat design that is correct is one with a displacement hull and just enough power to move it at displacement speed. It's certainly one viable option. But it's by no means the only viable option.