How Much Horsepower

[Marin]

No, the nose of the floatplane wil rise even if you hold down elevator. Particularly right at idle when the propwash is minimal. Eventually when propwash becomes very strong the upward force of air against the down elevator will begin to counter the hydrodynamic pressure that's building against the bottoms of the floats although even with full down elevator the plane will still come up on the hump with a lot of power. It gets a little ugly after that, though, if you continue to apply down elevator. It's all about the hydrodynamic pressure on the floats. I learned this from my good friend Jay Frey who for decades was president of the float division at EDO.

Our boat has no more power than any other twin GB36 and less than many, and at the time the props had the same diameter and pitch as other GB36s with similar power and 4-bladed props. So I suspect that if one did the same "test" I did with another GB36 they'd have the same results.

I think it's inevitable, frankly. You move something faster through the water and you're going to generate more hydrodynamic pressure against the hull. And Newton says for every action there is a re-action.

The Gikumi that I was on a few weeks ago does exactly the same thing. This is the boat I've posted some shots of recently. We were idling along, a big herd of Pacific Whitesided dolphins came along, and the skipper started opening the throttle of the boat. I was standing on the bow and I felt the bow start to lift the moment the engine speed began to increase. And the Gikumi is the very definition of a displacement boat.

 

[THD]

I am not sure what happens a idle or just above idle speeds, but as you approach hull speed, the bow rise is actually a stern fall, the stern "squatting" as the boat attempts to overcome the increasing drag created by larger bow waves. In a planing hull, this is when the boat "gets over the hump" i.e. transitions from in the water to on top of the water. If you have ever been in a planing boat that has toruble getting on plane, the bow goeas way, way up as the bow wave increases but the drag creates a "hole" in the water at the stern.

 

[psneeld]

No, the nose of the floatplane wil rise even if you hold down elevator. Particularly right at idle when the propwash is minimal. Eventually when propwash becomes very strong the upward force of air against the down elevator will begin to counter the hydrodynamic pressure that's building against the bottoms of the floats although even with full down elevator the plane will still come up on the hump with a lot of power. It gets a little ugly after that, though, if you continue to apply down elevator. It's all about the hydrodynamic pressure on the floats. I learned this from my good friend Jay Frey who for decades was president of the float division at EDO.

Our boat has no more power than any other twin GB36 and less than many, and at the time the props had the same diameter and pitch as other GB36s with similar power and 4-bladed props. So I suspect that if one did the same "test" I did with another GB36 they'd have the same results.

I think it's inevitable, frankly. You move something faster through the water and you're going to generate more hydrodynamic pressure against the hull. And Newton says for every action there is a re-action.

The Gikumi that I was on a few weeks ago does exactly the same thing. This is the boat I've posted some shots of recently. We were idling along, a big herd of Pacific Whitesided dolphins came along, and the skipper started opening the throttle of the boat. I was standing on the bow and I felt the bow start to lift the moment the engine speed began to increase. And the Gikumi is the very definition of a displacement boat.

I agree...most boats except the extreme hull designs which I have less experience with will rise as you add power.

 

[Nomad Willy]

But the bow won't rise until one gets to within about a knot of hull speed. The gentleman that thought hull speed was at that point certainly wasn't referring to a speed or throttle setting "just above idle".
PS ... When I say "bow rise" I'm referring to the fore and aft angle of the whole boat ... Not just the bow. Clearly the bow rises and the stern drops.
I'm still hoping someone will make a run and observe the speed.
And Marin I was talkIng about the horizontal stab ... Not the elevator.

 

[Marin]

Last time I looked the elevator is attached to the back of the horizontal stabilizer and its power will way overcome any effect air moving over the stabilizer portion of the empennage might have. So if the air moving over the stabilizer puts a downward force on it, putting the elevators down will negate the stabilizer's downward force by a major big factor.

 

[Marin]

But the bow won't rise until one gets to within about a knot of hull speed. When I say "bow rise" I'm referring to the fore and aft angle of the whole boat ... Not just the bow.

I think it would be quite a trick to raise the bow but not change the fore and aft angle of the boat. The bow and the rest of the boat are sort of attached to each other.

The point you seemed to be making earlier is that the bow will not rise until hull speed starts to be exceeded. I agree that the attitude of the hull will change at that point as the hull begins to dig a deeper hole in the water and the stern sinks into it. But if your point is that the hull will ride perfectly level, or at static trim, unitil that speed is reached, from my observations it doesn't.

As soon as power is added and the hull begins to move forward, its forward motion against the water begins to generate hydrodynamic pressure against the hull. It's physics--- it has to. And that pressure is going to move something. Some of that pressure is disspipated back into the water itself. But some of it is going to lift the hull. Not a lot, but some. Hence what I observed in my crude pencil test and what I felt on the bow of the Gigkumi. In both cases, the bow began to rise the moment the boat began to accelerate even though that rate of acceleration was very slow.

More speed, more pressure, bow is forced up.

How all this relates to the wave produced by the hull and why it "squats" as the speed increases I have no idea. All I'm saying is that as soon as a hull begins to move through the water there is pressure against the parts of the hull that are meeting the water and this pressure begins to lift the hull where the pressure is greatest, which on a conventional design is the front end. So the bow comes up. Our GB does it, seaplanes do it, our 17' Arima does it, etc.

 

[Nomad Willy]

As to the power requirement consider carefully how much of the power you used on the old engine and how that loaded the engine. How important engine loading is, is not really clear but engine manufacturers have the best information .... especially the engine you will have. I personally prefer 55 to 65 % of WOT determined by a percentage of the maximum fuel that the engine can burn at rated rpm. Get that max fuel burn number from the engine manufacturer. A 120 Lehman is 6 gph for example. As I recall most of our engines are most efficient at about 1800 to 2000 rpm. Over propping may produce even more efficiency but has it's own problems. Many do it though. Running your engine about 600 rpm down from it's rated speed would be a good load to shoot for if you can attain max rater rpm .... And you should be able to do so. It's quite likely a smaller engine would be better for you as most boats are over powered. I think a smaller engine running at a greater load is better but many disagree w me. I also think one needs very Little (if any) extra power for bad weather ect. The most objective people to talk to about this aren't on the forum ..... Manufacturers!

 

[Nomad Willy]

Never observed what the bow does attitude wise just above idle. No reason to either. But the bow does reach a point 1/2 to 1 knot below hull speed where it distinctly rises. That's the event I'm talking about. Quite a number of people think the bow rising event is the magic hull speed event ... Not so but it could be even a more important speed to be aware of.
But different hulls will probably have the event at different percentages of hull speed. A banana shaped hull having less pitch stability than a boat more typical of most trawlers w relatively plumb stem and full bows w large, flat and straight aft sections. They resemble cars w long wheelbases. So ther'e may be too many variables to apply specific numbers to specific events but the specific bow rising speed for a specific boat could be close enough to be an excellent rule of thumb. I'm hauling my boat in a few days so I'll not be able to make any specific observations but any number of boat owners here could play this out. In a week or so I'll be going back to Alaska for a month or so.

 

[JAT]

I know this really doesn't fit here...but I bought a 2004 Dodge Ram 3500 Dually with the 5.9L Cummins Diesel....thought I was being patriotic...turned out the truck was "Hecho en Mexico".... It was a new Cummins engine...but in the 8 years that we owned the pickup, I never had a single problem with it. Engine, tranny or body....sold the truck for almost half what I paid for it..... Being reman'd or recon'd in Mex under the guidance of Cummins.... I would have no problem buying that engine..... Would prefer one done in the US simply because I am a tad xenophobic....

 

[ksanders]

As a FYI to the OP.

There was a great article in Passagemaker magazine a year or so ago about the Cummins reman facility in Reynosa Mexico. You might contact passagemaker and get a copy of that article.

When we repowered our boat we opted for the Cummins Reman engines, in part based on that article. Their facility is pretty impressive.

 

[FF]

.turned out the truck was "Hecho en Mexico"...

The bigger problem is the BOAT engine is very different from the Truck engine , tho both are similar.

 

[sunchaser]

The Cummins 6B NA reman is IMHO the best way to go. PMM last year had a good article on the Mexico rebuild plant, very impressive. Talk or e-mail with Tony Athens about service and warranty in your area. Suggest you get a thread going on boatdiesel.com on this same subject.

 

[Nomad Willy]

Marin,
I think I've figured out the bow rising issue. I suspect what I've been saying about the bow rising applies to displacement hulls only. I've only used that observation on full disp boats and the concept of hull speed is somewhat meaningless on any other type so I was sharing that w you not knowing other hull types don't respond the same way. I've heard the bow rising concept on BoatDesign and other places. I'm not the only one that has had that concept. So maybe that explains it??

 

[Marin]

Marin,
I think I've figured out the bow rising issue. I suspect what I've been saying about the bow rising applies to displacement hulls only.

You may very well be right. We don't have a displacement hull so what I've observed applies only to the hulls I've observed it with. I readily admit I don't know diddley about hull design, I just know what I see.

 

[rwidman]

You might want to do a search for recent articles by Steve D'Antonio. He wrote recently (in Passagemaker I think) that virtually all displacement hulls are way overpowered. He is pretty compelling on the issue. ..........

I read Mr. D'Antonio's articles and he is certainly very knowledgeable. I think sometimes he goes a little over the edge on certain issues. By stating that "virtually all displacement hulls are way overpowered", he seems to be saying that the engineers who design these boats don't know what they are doing. That puts him at odds with dozens of boat manufacturers. It makes me wonder.

 

[psneeld]

I read Mr. D'Antonio's articles and he is certainly very knowledgeable. I think sometimes he goes a little over the edge on certain issues. By stating that "virtually all displacement hulls are way overpowered", he seems to be saying that the engineers who design these boats don't know what they are doing. That puts him at odds with dozens of boat manufacturers. It makes me wonder.

Marrketing ultimately designs the boat, the NAs or enginerrs just make it work.

Got to know the NE Regional VP for marketing pretty well when I worked for Sea Ray/Marine Max...amazing how the process is so convoluted. That's why there's so many so-so boats with such a strong following.

 

[THD]

PSneeld-you are certainly right on the marketing angle. However, when it comes to power, I think the issue is the reverse for newer trawlers. The marketing angle is lower power, fuel efficiency. But you can really see the overpower in the big planing, or sportfishing, market. I saw a 63' sportfisher at a show last year with twin 1500 HP MTUs! I looked around the engine room and boy is a mechanic going to charge you for crawling around those babies. Like no work space. Top speed about 48 knots. The problem is, how many days can a 63' boat go 45 knots on the ocean? Or anywhere else for that matter? Fuel burn was massive and range at speed was like 250-300 miles. But, marketing says that boat has to have that speed capability whether actually useful or not.

 

[Nomad Willy]

THD wrote:
"The marketing angle is lower power, fuel efficiency." Disagree big time. Trawlers are still very over powered just like S DeAntoino says. But the biggest problem with efficiency is the hull designs. They are designed for space, stability and the ability to carry weight. Most of our trawler boats are going at displacement speeds but don't have displacement hulls.

Fuel is still too cheap. If it was too expensive trawlermen would be scrambling to snatch up the few boats that are displacement like Krogens and Willards. When fuel gets expensive people will start repowering w smaller engines and avoid buying boats that are'nt full displacement. Until such a time fuel will be too cheap and boats far less efficient than they could be.

 

[ksanders]

The whole overpowered thing is really based on what the market wants.

The fact is that most people that can buy a $500K+ boat work really hard and have less free time than they'd like. The bigger engines are a result of meeting the demand for speed from potential buyers of these new boats.

If we look, the market for full displacement boats has really morphed into a market for retired people that want to explore the world. Thats why the krogens, and nordhavns are doing so well. These are expedition boats, vs the boats purchased by still working people are recreational boats.

 

[FF]

By stating that "virtually all displacement hulls are way overpowered", he seems to be saying that the engineers who design these boats don't know what they are doing. That puts him at odds with dozens of boat manufacturers. It makes me wonder.

Engines are bought by the purchasing folks , looking for the "best deal".

Oversized is "boob bait for the bubbas" that have ZERO concept of diesel engine operation.

What HP the NA specified for a certain service , and what was cheap and availible (usually on credit till the boat is sold) may differ .

Custom boats are usually far better done than cookies, even $2,000,000 cookies.