Stability of Slow Cruising a Deep Vee Hull in Rough Water

[goboatingnow]

if you have already decided everything and in fact have actually done it, why are you here asking for feedback, the horse has bolted.

I dont believe Ive insulted anyone, ( I still suggest it makes no sense)

Your comments re diesels, belies the actual proof in the market, all boats now tend to be fitted with higher performance turbo diesels, even sailboats. why , better fuel economy , better emissions, better power to weight.

Why would you want 8000 hours, at an average use of annual 150-200 hours thats 40 years of use!!!. Any of the modern higher performance diesels are all capable of outlasting your usage if maintained.

The Perkins 4108 is derived from an industrial engine, designed to run 24/7, thats a very different criteria from leisure boating use. ( Im not arguing its not a good engine, but it is rather dated).We have stacks of them in older boats all over the place this side of the pond, I even saw them in Peterborough

Lastly surely if you want a displacement mo-bo, its better , cheaper and easier to sell the bayliner and buy one. The repower you are doing ( or have done) would buy a lot of gas for the chevy.

[David M]

Originally Posted by Astral Blue

David, thanks for the insight. With respect to the stability versus seakeeping terminology, I find myself conflating the two on a regular basis. You're correct. They are different but closely related.

With respect to the hull design, my hull does not have a wide flat area with little deadrise, as you described. That's what puzzles me with respect to what extent it will behave like a displacement hull. Most displacement hulls (with squared chines, that is) I've see have a greater bowrise, but resemble its pitch and angle until the stern. Where they differ at the stern is the lower and relatively flat tapering, whereas my hull ends in a steeper vee angle.

It is a slow (inefficient) planing hull but provides a very smooth ride; and I attribute it to the lack of flat surface. I could be wrong.



Yes, it does have a considerable amount of windage; and its center of gravity needs considerable improvement.

One thought I have been entertaining is to add a water ballast to increase its seakeeping ability. Any thoughts?

Correct, more deadrise aft helps make it a more comfortable ride on a plane.

Don't add water ballast. Weight is the enemy of speed and efficiency.

I really think you need to consider a different boat designed from the beginning for better efficiency at lower speeds. Modding what you have is not going to get you a very efficient, well designed boat. The boat you have was designed for planing and that is what its hull shape does best. Boats are best designed from the ground up for a specific purpose.

[Dulcesuenos]

Someone mentioned getting to hull speed on this w little hp as they did on a sailboat but their sailboat and any non planing hull is different than a hull designed to plane. Our old byliner would throw a pretty good wake at 7 knots. Our sailboat throws no wake at same speed. The bayliner would also start to plane as little as 13-14 knots dunno about yours but ours was actually great on fuel. I believe we averaged 5-6 mpg with the ability to go fast. The storm we were in did burn a crapload of gas in a relatively short time. Though. In rough stuff going slow or anchored she was a rocky rolly mess. But so would most monos

[David_Old_Jersey]

Originally Posted by goboatingnow

Lastly surely if you want a displacement mo-bo, its better , cheaper and easier to sell the bayliner and buy one. The repower you are doing ( or have done) would buy a lot of gas for the chevy.

+1

But nonetheless I can see that if the original engine has gone majorly pop (requiring a total repower), the boat is otherwise sound and using it as essentially a houseboat that can be used around the bay and also moved from place to place (on nice days / with a careful eye on the weather) works well for OP - then why not.

Over here an old boat like that with a blown engine is worth next to nothing - I presume same elsewhere....so IMO worth a go (if the limitation on the results still work well), accepting that resale will be next to nadda. If OP had yanked a perfectly good engine, then (unless had been sold for a very good price) I think he would need head examining as would have been far and away better off (in cash and results) to have sold and bought something else. Or have just throttled back a bit whilst still accepting that she would still be going faster than wanted.

But it's boats - stuff don't always have to make sense .

[Tony B]

In the Original Posting, AstralBlue stated "My typical cruising speed is 7-8 knots. My new engine will be able to give me a 7-8 knot cruising speed comfortably." If you were compfy then, nothing should have changed in the way of stability by swapping engines. The boat characteristics should not change at that speed from then to now.

[Astral Blue]

Originally Posted by David_Old_Jersey

+1

But nonetheless I can see that if the original engine has gone majorly pop (requiring a total repower), the boat is otherwise sound and using it as essentially a houseboat that can be used around the bay and also moved from place to place (on nice days / with a careful eye on the weather) works well for OP - then why not.

Over here an old boat like that with a blown engine is worth next to nothing - I presume same elsewhere....so IMO worth a go (if the limitation on the results still work well), accepting that resale will be next to nadda. If OP had yanked a perfectly good engine, then (unless had been sold for a very good price) I think he would need head examining as would have been far and away better off (in cash and results) to have sold and bought something else. Or have just throttled back a bit whilst still accepting that she would still be going faster than wanted.

But it's boats - stuff don't always have to make sense .

The original post was asking people to share their experience/insight as to whether a deep vee hull would handle well in rough seas at low speeds. It was not weather this project would be financially feasible. But to answer the question, yes it is. And when breaking down the cost, it's a no-brainer.

My boat had a perfectly good engine with 400 hours. I managed to get a great price for it when I sold it. If I sold the boat, it would have gone for $8000 to $9000 in this market. My repower (parts and labor included) will cost $10000. I could not get a Diesel powered cruiser (with a new engine, nonetheless) in the condition my boat is in for anywhere close to $20000! My boat's interior is in immaculate condition, has been upgraded with many creature comforts and wood appointments, and has been refitted extensively with new wiring, plumbing, sanitation system, etc.. It is a very comfortable cruiser and has a layout that cannot be found in many boats. I have no intention of selling it; and by the time the engine is exhausted, I will have a lot of gray hair on my head!

During this repower, the transom has been rebuilt, a new fuel tank has been installed, outdrive has been repainted, and all of the hoses, bellows, stringers, blocks, motor mounts, and seals have been replaced. With an extensive refit as this, there is no way a cruiser in this shape and such appointments will be available for even close to what I'm paying. So yes, it is a no-brainer to spend less than $20K for a cruiser I can't even purchase for $30K.

This is not a houseboat intended only for protected waters. While it isn't a blue water boat, it will have the ability to cruise in all parts of the delta and the bay; and hopefully outside the Golden Gate bridge along the coast under good conditions.

Originally Posted by Tony B

In the Original Posting, AstralBlue stated "My typical cruising speed is 7-8 knots. My new engine will be able to give me a 7-8 knot cruising speed comfortably." If you were compfy then, nothing should have changed in the way of stability by swapping engines. The boat characteristics should not change at that speed from then to now.

I'm hoping this will be the case. I just wanted to get insight as to whether the low speed can maintain stability in rougher conditions. Most people who have this boat take it on plane to get the bow up, because it has limited bowrise. They "surf" the waves and do a lot of throttle work to avoid burying the bow.

[David_Old_Jersey]

Originally Posted by Astral Blue

The original post was asking people to share their experience/insight as to whether a deep vee hull would handle well in rough seas at low speeds. It was not weather this project would be financially feasible. But to answer the question, yes it is. And when breaking down the cost, it's a no-brainer.

It's a forum, you get to start the thread and then to suck out everything you need - but you don't get to pick and choose how the conversation goes......nor can you avoid anyone pointing out any elephants in the room - in this case that includes the cost vs the benefit. That might not be relevant to you, but could be for others thinking similar - and that's half the purpose of Forum threads.

But glad the figures work for you ....all I see is $20k being flushed down the toilet on a beast that will be neither fish nor fowl.....

Anyway, I do wish you well.

[Tony B]

Ed

Since I know absolutely nothing about you and your knowledge of boats I will err on the safe side and throw this in:
Your most MPG will probably be in Idle and no one wants to put-put at 5 MPH. Saying this, your next major hurdle will be at hull speed. If you stay at just below your hull speed (6.7 Kts) and travel at say 6.5 kts, you will be saving a lot of fuel - I think. I don't know your boat weight or any thing else about your motors or boat but the same rules still apply. Once you approach hull speed and go slightly faster, your fuel consumption rate will increase and your MPG will drop dramatically. It's not until you get up into a plane that your fuel consumption levels out with an increase in speed hardly noticable - fuel wise. In your case the last part of this is irrelevant because you wont be planing but you could be burning a lot of fuel at 8 Kts.
Going from 6.5 to 7.5 kts could possibly cost you and extra MPG or more. Some people are budgeted for that and some aren't. Just thought you should be made aware.

[Pete7]

Originally Posted by Tony B

In the Original Posting, AstralBlue stated "My typical cruising speed is 7-8 knots. My new engine will be able to give me a 7-8 knot cruising speed comfortably." If you were compfy then, nothing should have changed in the way of stability by swapping engines. The boat characteristics should not change at that speed from then to now.

It will be interesting to here the final result of this. However, I suspect that whilst 48hp in a sleek displacement hull designed for that hp would give a cruising speed of 7-8 knots, I am not sure it will in a planing hull. Reason? the planing hull has a flat transom which the stern drives bolt to. When planing the transom is out of the water and need not be considered. However, at slow speeds that same transom is going to create a huge amount of drag, which will need hp to overcome.

I agree with David comments and if it were mine I would drop a cheap V8 in and flog it to buy a displacement hull boat. It would be much more sea kindly in bad conditions.

Pete

[Tony B]

Originally Posted by Pete7

.... When planing the transom is out of the water and need not be considered. However, at slow speeds that same transom is going to create a huge amount of drag, which will need hp to overcome.
Pete

Pete, I am not familiar with planing hulls but I would think that this would not be a problem below theoretical hull speed. I am under the impression that to plane you have to climb your own bow wave and below hull speed you have not yet created that bow wave in the proper position to climb.
I really dont know much about this so feel free to correct me where I am wrong.

[Astral Blue]

Originally Posted by Pete7

It will be interesting to here the final result of this. However, I suspect that whilst 48hp in a sleek displacement hull designed for that hp would give a cruising speed of 7-8 knots, I am not sure it will in a planing hull. Reason? the planing hull has a flat transom which the stern drives bolt to. When planing the transom is out of the water and need not be considered. However, at slow speeds that same transom is going to create a huge amount of drag, which will need hp to overcome.

I agree with David comments and if it were mine I would drop a cheap V8 in and flog it to buy a displacement hull boat. It would be much more sea kindly in bad conditions.

Pete

Hi Pete... Thanks for chiming in. You brought up a point worth investigating. I posted a few images of full displacement hulls. If I'm not mistaken, there doesn't seem to be a feature on the transom that minimizes drag. The propeller is of course connected to a shaft and protrudes from the keel, but whether that feature makes a difference in the drag is something I have found little data to support.

For what its worth, I should disclose my boat has an 8 foot beam. Most 28 foot boats have a 10 foot beam; but one of the reasons I chose my boat was its narrow beam -- as it would make it trailerable without needing a wide load permit. Now its narrow beam works in my favor toward fuel economy but becomes a drawback with respect to stability.

I also notice many sailboats have a flat transom as well; and they are full displacement hulls. I'm guessing the transom shape might not play a role at hull speed (as Tony mentioned)...

[Tony B]

Originally Posted by Astral Blue

Hi Pete... .. If I'm not mistaken, there doesn't seem to be a feature on the transom that minimizes drag. .......
..... I should disclose my boat has an 8 foot beam. Most 28 foot boats have a 10 foot beam .....Now its narrow beam works in my favor toward fuel economy but becomes a drawback with respect to stability.

I also notice many sailboats have a flat transom as well; and they are full displacement hulls. I'm guessing the transom shape might not play a role at hull speed (as Tony mentioned)...

What minimizes drag is the entire shape of the hull. Sometimes looking at a boats stern can be deceiving. Most sailboats have what appears to be a flat stern when in reality, it is canoe shaped at the waterline. Next time in a marina, look at a sailboat stern closely and you will notice that at the water line the boat comes to a point. Its not until you get above the waterline that you see the flat stern. What makes it slip through the water with ease is that as it moves through the water and pushes the water aside, the canoe shape in the stern allows the water to flow back around the sides and more or less push the hull forward. A flat stern creates turbulence.
Another thing, correct me if I am wrong but I believe the narrower the boat, the less rocking. With a wide beam you have your buoyancy spread out over a good portion of a wave so on a wide boat one side will ride up the wave while the other is floating on the bottom of the wave a good distance away. With a narrow beam, the wave tend to pass under the boat quicker but the boat will tilt less. The narrow beam gives a more sea kindly ride.
I am curious how this will all play out when you get it in the water. My money is saying that below hull speed the planing hull will not be that much of a detriment and the boat will ride just fine.
If nothing else, the boat should behave at 6.5 knots with the new diesel the same way it did at 6.5 knots with the old motor. I dont know how much it would cost but I would also think that trim tabs should help also

On the other side of the coin, a displacement hull will never plane. It will porpose up and down if you could get it to go fast enough.

[Tony B]

Ed:

1) Is your boat considered a planing hull or a semi-displacement hull?
2) I am dieing to see a picture

With diesel, you should have a really good traveling range. And...........pretty maintenance free.
So, how big was your old fuel tank and are you still using it. I would guess that a 50 Gal tank would get you about 300 to 350 miles. If you went down to a 50 gal tank and the smaller size of your motor, I would think you have captured some extra room.

[JayCall]

Maybe I am offbase here as most repsonses concern the power and fuel usage. As to stability (or seakeeping as previously noted), the planing hull is going to react quite differently in heavy water than a full-displacement hull. The former is desigend to be most efficient, and most comfortable, riding on plane, the latter with the hull fully in the water. At displacement speeds, the flat stern and hard chines tate provide lift when planing, still provide lift at low speeds. That lift extends virtually the entire beam of the hull. In otherwords, the buoyancy is spread pretty evenly across the entire width of the hull. Compare to a full displacement where the hull volume decreases substantially and usually pretty quickly as you near the turn of the hull. Add to that that the center of buoyancy in a planing hull is generally a good bit higher than in a displacement hull. In a displacement hull, the hull shape deflects a good deal of the vertical force during rolling. In a planing hull, the flat after sections and the hard chines bear the full brunt of those forces. In short, a planing hull, at hull speeds, will roll easier and more quickly. You get the feeling of a "snap-roll" in quarter to full beam seas. You also get a lot of yawing in a following seas for the same reasons. Makes for hard steering. And an uncomfortable ride.

[goboatingnow]

Originally Posted by JayCall

Maybe I am offbase here as most repsonses concern the power and fuel usage. As to stability (or seakeeping as previously noted), the planing hull is going to react quite differently in heavy water than a full-displacement hull. The former is desigend to be most efficient, and most comfortable, riding on plane, the latter with the hull fully in the water. At displacement speeds, the flat stern and hard chines tate provide lift when planing, still provide lift at low speeds. That lift extends virtually the entire beam of the hull. In otherwords, the buoyancy is spread pretty evenly across the entire width of the hull. Compare to a full displacement where the hull volume decreases substantially and usually pretty quickly as you near the turn of the hull. Add to that that the center of buoyancy in a planing hull is generally a good bit higher than in a displacement hull. In a displacement hull, the hull shape deflects a good deal of the vertical force during rolling. In a planing hull, the flat after sections and the hard chines bear the full brunt of those forces. In short, a planing hull, at hull speeds, will roll easier and more quickly. You get the feeling of a "snap-roll" in quarter to full beam seas. You also get a lot of yawing in a following seas for the same reasons. Makes for hard steering. And an uncomfortable ride.

Well this is contradicted by deep V RIbs which are planing and are excellent heavy weather machines. You should read DAg Pikes book on mobo heavy weather techniques. He prefer good planing hulls. ( and open the throttles)

Dave