Stability of Slow Cruising a Deep Vee Hull in Rough Water

[David_Old_Jersey]

Ok, I lied about that being my last post - sue me .

Originally Posted by Tony B

As for my country, I have pride in it as I do myself. Enough pride to display my profile and not hide behind the anonymity of the internet.

To make a statement like that is either a very subtle and very self aware parody of a national stereotype (and for which I take my hat off to you but only if it is your own nationality that you are making fun off, otherwise it's a bit rude )...............

[Astral Blue]

Originally Posted by Tony B

[/B]
I had no idea there was that much of a weight difference.
Shaving 1,000 lbs from the stern of your boat will definitely have some effect on stability and planing - even if not a full plane, just the dropping of the stern prolly wont be as severe.
I should have such problems. That means that if you wanted to rebalance the boat you would prolly have to take on around another 100 gals of drinking water. Damn. LOL. With the added space you now have, that should be easy to do. Custom fit fiberglass tanks.
Anyway, you wont know for sure until you do your test runs. Much easier to add weight than to remove weight. A Naval Architect should be able to tell you exactly where to place the weight to achieve the stability performance you want.

Tony, here is my thinking... Stern-heavy boats are horrible candidates for planing. Shaving 1000 lbs. from the stern is equivalent to 1/6th of my boat's overall new weight! That's pretty significant. With that weight distribution and some help from the trim tabs, I'm very curious as to how much of the hull will lift up out of the water. Whether this provides stability is in itself another item worth investigating further.

[Tony B]

Ed

Soooooooooooooooooooooo, when do you splash so we can all find out?
I'm curious as all hell right now.
BTW, I am not at all familiar with the west coast so I'll ask out of ignorance.....do you have any navigable waterwats nearby?

Oh, also thought of this....your boat is 1000 lbs lighter, so trailering it would be a lot easier on your vehicle. I often think about going to a trailerable boat. So many really cool landlocked lakes that I wouldn't mind driving to.

[Astral Blue]

Originally Posted by Tony B

Ed

1. Soooooooooooooooooooooo, when do you splash so we can all find out?
I'm curious as all hell right now.

2. BTW, I am not at all familiar with the west coast so I'll ask out of ignorance.....do you have any navigable waterwats nearby?

3. Oh, also thought of this....your boat is 1000 lbs lighter, so trailering it would be a lot easier on your vehicle. I often think about going to a trailerable boat. So many really cool landlocked lakes that I wouldn't mind driving to.

1. The project has been making slower progress than I anticipated. My mechanic is moving at a snail's pace. If I'm lucky, a late June/early July splash date might happen.

2. I primarily cruise in the California Delta, which has over a thousand miles of protected navigable waterways. Here is a glimpse of the area I mostly cruise in. The photos were taken in the north part of the delta. And then of course, there is San Francisco Bay...which is a 5+ hour cruise from my marina.

3. Weight has never been an issue when trailering my boat. I have a Ford F350 Diesel, which handles the weight well. I have to admit, though. Trailering this boat is something I'd rather not do. It is a huge beast when it is out of the water. I have trailered 24 foot boats and they have been relatively painless. But a 28 footer (with a flybridge no less) is far more intimidating.

[Tony B]

Great pics of the Delta. Thanks.
Hope something comes up and you splash sooner.

[Pete7]

Originally Posted by Tony B

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.

Tony, I think you will find it has a major effect and lots of drag that will:

a. Need lots of power to overcome compared to a sleek yacht hull.

b. Create a huge wash, more wasted power.

Pete

[goboatingnow]

Originally Posted by Pete7

Tony, I think you will find it has a major effect and lots of drag that will:

a. Need lots of power to overcome compared to a sleek yacht hull.

b. Create a huge wash, more wasted power.

Pete

That in my experience is a gross exaggeration. At any speed below theoretical max displacement hull speed, planing hulls do not generate much wash. The reason that a lot of wash is seen, is that because of the power they have they are habitually driven above hull speed but below planning speeds. This is where they are at their most inefficient and most of that inefficiency is seen in the huge wash.

Many yacht hull arnt that sleek.

[Pete7]

Originally Posted by Astral Blue

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.

The feature on the transom that minimizes drag is the underwater canoe shape, as compared to a planing hull which is like a bar door being dragged sideways through the water.

Originally Posted by Astral Blue

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)...

Its the underwater transom shape that counts, ignore the above water shape.

Pete

[Tony B]

I sure wish his mechanic would hurry up. I'm dieing to find out the results. He has a FloScan meter so he would be able to get detailed results. I believe he collected data with the FloScan prior to the swap-out so now he will be able to compare.

[Tony B]

I think we get side tracked here from time to time. We keep comparing it to a full displacement hull when what we should be doing is compare it to itself. The boat has not changed. It is the same boat. So, the real question is can a 48HP diesel engine push this boat at 7 MPH? I think a 9.9HP outboard could push it to that speed.
My guess is that the boat weighs in at 6 ot 7,000 lbs. That equates to 3 1/2 tons.
Frome everything I read, it takes 2-3 HP per ton to push a vessel at almost hull speed. Lets assume the 2 is for an easy moving hull like a full displacenment so we wont use it. Lets go with the 3 HP/ton which would be closer to what we need and we get only 10.5 hp required. Even if we went to 5 HP/Ton it would still only need 17.5 HP to move it to about 7MPH. He has 48HP in there now. The boat is the same boat, nothing has changed except the motor and less weight. Under the worst of conditions he can replace it with lead, steel, concrete or 100 gal worth of water tanks or 500 cans of Campbells Soup. That's under the worst of conditions. Under the best of conditions, the stern is now much lighter and most claim that that model has too heavy of a stern to begin with. Given that, he migh be better off. In reality, he will prolly end up somewhere in the middle and still have enough HP to spare.
If you dont think a 9.9 HP OutBoard could push that boat at 7 MPH then the discussion is over and all we can do is wait for the in-the-water test results.

[Pete7]

It will need 48 hp. 3hp to drive the boat and 45 hp to run the alternator to power the auto pilot to stop it wandering all over the place.

He said she has shafts, so that's rudders designed to work at 20 knots. they will be fun at 5 knots.

Pete

[Tony B]

What makes this thread so interesting is that none of us are Naval Architects and we are just speculating.
He currently runs the boat now and has been running it at around 7 MPH so I dont see anything changing there. If it was squirrelly before the motor change it will be after the motor change. If it wasn't squirrelly before the change it shouldn't be now except for the weight change, and that could be easily corrected by adding weight.
Oh well, I will patiently wait for this project to finish.

[Astral Blue]

Originally Posted by Tony B

I think we get side tracked here from time to time. We keep comparing it to a full displacement hull when what we should be doing is compare it to itself. The boat has not changed. It is the same boat. So, the real question is can a 48HP diesel engine push this boat at 7 MPH? I think a 9.9HP outboard could push it to that speed.
My guess is that the boat weighs in at 6 ot 7,000 lbs. That equates to 3 1/2 tons.
Frome everything I read, it takes 2-3 HP per ton to push a vessel at almost hull speed. Lets assume the 2 is for an easy moving hull like a full displacenment so we wont use it. Lets go with the 3 HP/ton which would be closer to what we need and we get only 10.5 hp required. Even if we went to 5 HP/Ton it would still only need 17.5 HP to move it to about 7MPH. He has 48HP in there now. The boat is the same boat, nothing has changed except the motor and less weight. Under the worst of conditions he can replace it with lead, steel, concrete or 100 gal worth of water tanks or 500 cans of Campbells Soup. That's under the worst of conditions. Under the best of conditions, the stern is now much lighter and most claim that that model has too heavy of a stern to begin with. Given that, he migh be better off. In reality, he will prolly end up somewhere in the middle and still have enough HP to spare.
If you dont think a 9.9 HP OutBoard could push that boat at 7 MPH then the discussion is over and all we can do is wait for the in-the-water test results.

To provide clarification, I attached a screen shot of boatdiesel.com's power required calculation for my boat. Note the calculation is for a planing hull and has all of the specifications about my boat and drive system indicated. To attain 8 knots, 15.5 horsepower is required at the prop shaft.

I'm on my way to have a talk with my mechanic. You and I and a pretty big contingency of interested people on the forums I've discussed this on are all awaiting the big splash. I'll be able to have a better idea with respect to timeline after I talk to him today.

Originally Posted by Pete7

It will need 48 hp. 3hp to drive the boat and 45 hp to run the alternator to power the auto pilot to stop it wandering all over the place.

He said she has shafts, so that's rudders designed to work at 20 knots. they will be fun at 5 knots.

Pete

Pete, your thoughtful contribution to this thread is quite valued.

As I indicated earlier, bow wonder is not an issue whatsoever at 6 knots. There is also a product called wonder fin, which has had a great proven track record at keeping bow wonder under control. I have not had to worry about it much. I only experience bow wonder at no-wake speeds, which constitutes only a small segment of my cruising time.

This boat has a Volvo Penta 280 outdrive. It does not have a shaft/rudder configuration. This model outdrive has been used on both planing, semi-displacement, and displacement hulls.

[Tony B]

(Astral Blue ....To attain 8 knots, 15.5 horsepower is required at the prop shaft. )

If it takes 1 gal of fuel to produce 18 HP with a diesel engine and only 15.5 HP required to travel at 8 kts that equates to 0.86 gals to go 8 Nm. Which means you should get 9.3 NM /gal or over 10 statute miles/gal.
Not too shabby.
I haven't sold my sailboat yet but I put money down on an older 36 Mainship Motoryacht. I would be thrilled to death to get 2 mpg out of it at only 7 MPH (Statute MPH)

[Astral Blue]

Originally Posted by Tony B

(Astral Blue ....To attain 8 knots, 15.5 horsepower is required at the prop shaft. )

If it takes 1 gal of fuel to produce 18 HP with a diesel engine and only 15.5 HP required to travel at 8 kts that equates to 0.86 gals to go 8 Nm. Which means you should get 9.3 NM /gal or over 10 statute miles/gal.
Not too shabby.
I haven't sold my sailboat yet but I put money down on an older 36 Mainship Motoryacht. I would be thrilled to death to get 2 mpg out of it at only 7 MPH (Statute MPH)

Tony, this information (at this point) is theoretical...and if it turns out to be consistent with the results I get after splashing, I will be ecstatic! I'll even be happy with a margin of error of 20 percent either way.

With that said, the actual nautical miles per gallon (I personally prefer to use gallons of fuel burned per hour at a cruising speed of X) is calculated to be somewhat lower than what was derived based on the horsepower needed calculation that was performed. I posted some calculations using a more comprehensive tool available on boatdiesel.com. It is the prop calculator, which takes into account any inefficiency the drive system places on the boat. I performed calculations for 2000, 2200, 2400, 2600, 2800, and 3000 RPM's. I attached the results.

Congrats on the Mainship! They are amazingly well designed in terms of layout. One nice feature I've only seen in Mainships is the actual stairway that goes to the flybridge instead of a ladder. All of the boats in this class and size I've seen use a ladder. The cabins are well laid out as well!