Whether Higher HP Engine is Worth It?

[Mark P]

I am looking at the Helmsman 38E with a 250 HP Cummins QSB 6.7. In published test ratings on this 30K Lb semi-displacement vessel, at 1K rpm it does about 5.8 knots, at 1500 rpm, it does 7.7 knots, at 2000 rpm it does 9 knots, at 2500 rpm it does 11.4. At WOT it does 14 knots. Not knowing how loaded the boat was, what could I reasonably expect the speed characteristics to be if I went with the 380 HP engine instead of the 250 HP? Trying to figure out whether the additional engine cost is worth it. Of course, fuel burn really goes up with each rpm increase, but setting that aside for now, any thoughts would be appreciated.

 

[dhays]

My guess (and it is only a guess) is that you won’t see much of an increase in the top speed. I suggest you ask the folks at Helmsman and see what they say. They should know.

Your fuel burn should be the same at the lower RPM with the two engines. The difference is that the 380hp will have a max RPM of 3,000 instead of the 2,600 of the 250hp and a different fuel map.

I don’t think you will running the boat at the top end of the engine rpm much at all. If you think you want more speed, then rather than a bigger engine you need to look at a different boats.

 

[Heron]

If you think you want more speed, then rather than a bigger engine you need to look at a different boats.

This.....

 

[DavidM]

It is all a matter of $$$- engine cost but mostly fuel, engine life and speed. You have to balance the three.


For example the 380 hp Cummins could be run to produce 250 hp and drive the boat to 14 kts and that would be a pretty good compromise among the three. You could also run the 380 hp Cummins to produce 320 hp and get a couple of knots more, but engine life would suffer.


David

 

[Lepke]

If you want more speed for less fuel, look at longer hulls. More hp in a short hull just leads to higher fuel usage. Even at idle. And the higher the engine hp, the more expensive the accessories like pumps, starters, turbos, injectors and so on...
In picking my current boat, I wanted economy, safety on the ocean and comfort over other considerations. Because I live aboard, I wanted laundry, full reefer, freezers dishwasher, etc. I have an 83' hull with early Detroit Diesels of about 200 hp. At 10 knots I get about 1.15 kpg and at 7 knots about 1.75 kpg but rarely run below 10.
Hull speed means everything when considering economy and engines.

 

[sharonandalex]

how much power

HI,

With semi displacement or full displacement hulls, more power usually means less miles per gallon. Any speed over hull speed is a waste of fuel unless you are trying out run a storm or perhaps make a lock or bridge on time. Our 50' semi displacement boat with twin Volvo 305 hp engines cruised easily at 10 mph or 8 kts at 1400 rpm which gave us from 1 to 1.5 mpg. Top speed of 12kts gave us 1/4 mpg at 2200 rpm. Idling at 6 kts gave us 2.5 to 3 mpg. The engines you have in the boat are large enough to give you good control in wind and current and wave. More power will not change that much. My rule of thumb is to have enough power to deal with currents, winds, and waves and maneuvering in the dock areas. I have seen 49' boats with 80 HP and for me that is too small. So relax, plan one mpg, and enjoy the trip.

Good luck with your adventure

 

[dhays]

A follow-up. I have a 43' boat with a hull that may not be as efficient as the Helmsman 38 if those number are accurate. I have a Cummins QSB 5.9 380hp engine. The original owner wanted the 380hp instead of the standard 330hp the builder was providing at the time. I have never needed or used that extra 50hp. I run the boat most of the time at 1,450 rpm to give me 7 knots with no wind or current (that has happened a couple of times). I burn 2 gph at that rpm.



I don't know anything about engines, but I would be just as happy with a 330 in my boat and if I had been the original owner, that is what I would have gone with. There is no disadvantage at all to having the 380hp engine for me, but I wouldn't have spent the money on the "upgrade".

 

[FF]

"I run the boat most of the time at 1,450 rpm to give me 7 knots with no wind or current (that has happened a couple of times). I burn 2 gph at that rpm."

This is how most displacement cruisers operate.

2 GPH is probably 35hp , perhaps 40 if the engine is quite modern .

Hardly a need for 300+ hp to operate at 30-40 hp.

If you want to tow water skiers , you will need the big engine + big fuel tanks.

 

[Mark P]

All, thanks for the great info. As I suspected, this is similar to my current planing vessel. I have 28 foot 2017 Sea Ray 280 SLX. I bought it new last year, and this vessel comes standard with a 350 HP Merc., with a top speed of 43 mph. The largest motor available (probably 480 HP) only gets a few mph faster according to Sea Ray specs. The larger motor probably allows this vessel to plane faster, which is no big deal for me - I am not comfortable taking this vessel over 20 mph in the low country with all the stumps, poles, shoals, and debris floating around here. This is great info as we plan to replace this "speed boat" with a trawler. We'll go the small engine route again. Thanks again for the info. Mark.

 

[OldDan1943]

Weight vs HP.
You may not be gaining anything.

 

[dhays]

Weight vs HP.
You may not be gaining anything.

On the Cummins, I think the weight is the same on the two engines the OP mentioned.

 

[seasalt007]

My current Nordic Tug 42' has a 540 hp QSC 8.3. My former Nordic Tug 42' had a 330 hp 6BTA.


Cruise speed for both boats is/was 8.5 kts.


The 330 hp would top out at 12 kts. The newer one at 18 kts with a clean bottom and light fuel and water.


I like the electronic 8.3 and all it's chrome pipes instead of rubber hoses but given my style of cruising don't really need more horsepower. And, yes it does burn more fuel at 8.5 kts than the 330...but not enough to bother me.

 

[Mule]

I might be wrong on this but as I recall the Cummins electronic engines you are discussing the only difference is software. Assuming this is true then buy the cheap. Have the need for more speed, have a tech come out with a laptop and he can give you the additional HP. For a tidy fee of course.

 

[Mark P]

Seasalt007 - when you say "light fuel and water", are we talking less than 1/2 in the tanks?

 

[seasalt007]

Seasalt007 - when you say "light fuel and water", are we talking less than 1/2 in the tanks?

Yes and a clean bottom.

 

[Hamrow]

Mark, there's a guy on here "Flyer50" that just went through this trade study on a new Helmsman 38E, seems like you could get more specific advice over in his thread and maybe in general under the Helmsman/Camano builder thread.
His thread was called "Helmsman 38E for May 2018 Delivery"

 

[warren]

Fuel use

Hi
you can get fuel consumption graph/charts for any Cummings engine at different rpm. You can use this to directly compare the additional consumption at the cruising speed. The higher hp engine will use more fuel. To me that would be a problem. Lower fuel consumption and longer range would be important to me.

The diesel wisdom is that you should run a diesel at a good load to avoid premature wear ,boreglazing etc issues. The smaller the engine the more you are running in the correct rpm range. (But apparently Cummins don’t seem to get a lot of low speed running issues reported).

If you have an electronic diesel option the improved fuel consumption , lack of smoke ,easy starting and many issues make these engines great unless the electronic are unreliable which I have no experience with on a Cummings , but I would bet they are reliable! Cheers warren

 

[Nomad Willy]

Probably a lighter narrower boat is a smarter better way to more speed than more hp.

 

[bayview]

IMO you will be marginally on plane at 14 KTS plowing and making big wake. trim tabs may help if available. After you add several thousand pounds of fluids and gear 14 Kts will probably not happen and you dont want to run WOT anyway.
Unless you plan to always cruise at 7 kts I would go with the bigger engine. You won't use more fuel unless you use all the power.