small boat opinions

[JLIMA]

I'm just looking for any opinions you may have on this idea i had. basically it's a 24' modified v hull with a 8.75' beam designed to be built with 2x frames on 18" centers and 1/2" skin with a displacement of 5,300lbs (loaded displacement). I intend to use her on semi-protected waters around Buzzards Bay and Vineyard sound off the coast of Massachusetts. I am currently intending to build in 75 US gallons of fuel tankage and power her with a 75hp Yamaha. Oh and i realize i didn't draw in any ports or windshields but that's more to a lack of my knowing what i want to do for an interior, but what ever i do i want to keep it simple.

[tom28571]

From the sketch, your boat looks fairly typical of many warped bottom small power cruisers with one exception. The transom is much too narrow. It appears to be about 60% of BOA. 85% to 90% would be more normal. Lesser transom beam does improve down wind/wave tracking but at the expense of aft buoyancy. Don't want extreme in either direction.

With the low buoyancy aft and relatively steep deadrise over much of the bottom, she will not be quick to get up on plane, especially with a 75hp, although you do not give displacement.

Otherwise it looks ok but it is only a sketch so few conclusions can be drawn.

[JLIMA]

Here are the hydrodynamics for her as is
Design length : 26.000 [ft]
Length over all : 26.322 [ft]
Design beam : 9.000 [ft]
Beam over all : 8.581 [ft]
Design draft : 1.500 [ft]
Mainframe location : 13.000 [ft]
Water density : 63.989 [lbs/ft3]
Appendage coefficient : 1.0000


Volume properties:
Displaced volume : 90.363 [ft3]
Displacement : 2.581 [tons]
Total length of submerged body : 24.661 [ft]
Total beam of submerged body : 7.305 [ft]
Block coefficient : 0.3344
Prismatic coefficient : 0.6526
Vert. prismatic coefficient : 0.4414
Wetted surface area : 155.39 [ft2]
Longitudinal center of buoyancy : 11.785 [ft]
Vertical center of buoyancy : 1.071 [ft]
Midship properties:
Midship section area : 5.615 [ft2]
Midship coefficient : 0.5124
Waterplane properties:
Length on waterline : 24.661 [ft]
Beam on waterline : 7.305 [ft]
Waterplane area : 136.49 [ft2]
Waterplane coefficient : 0.7576
Waterplane center of floatation : 11.111 [ft]
Entrance angle : 23.202 [degr.]
Transverse moment of inertia : 454.83 [ft4]
Longitudinal moment of inertia : 4899.4 [ft4]
Initial stability:
Transverse metacentric height : 6.104 [ft]
Longitudinal metacentric height : 55.290 [ft]
Lateral plane:
Lateral area : 29.768 [ft2]
Longitudinal center of gravity : 13.294 [ft]
Vertical center of gravity : 0.857 [ft]

I'm not looking to win any races of beauty pageants or races with her, I just want a top speed of around 15 knts which is semi-displacement speed from what i understand. Considering that is there anything else you could recommend besides widening the transom? I've built a few boats but never designed one so any input is welcomed.

[JLIMA]

here is another design i was throwing around she's 4' smaller and has the wider transom and her hydrodynamics are as follows ...and again the prospective motor is the same 75 hp Yamaha.

Design length : 22.000 [ft]
Length over all : 22.051 [ft]
Design beam : 7.000 [ft]
Beam over all : 7.073 [ft]
Design draft : 1.250 [ft]
Mainframe location : 11.000 [ft]
Water density : 63.989 [lbs/ft3]
Appendage coefficient : 1.0000


Volume properties:
Displaced volume : 61.850 [ft3]
Displacement : 1.767 [tons]
Total length of submerged body : 20.745 [ft]
Total beam of submerged body : 6.417 [ft]
Block coefficient : 0.3717
Prismatic coefficient : 0.6364
Vert. prismatic coefficient : 0.5059
Wetted surface area : 115.87 [ft2]
Longitudinal center of buoyancy : 9.522 [ft]
Vertical center of buoyancy : 0.878 [ft]
Midship properties:
Midship section area : 4.685 [ft2]
Midship coefficient : 0.5840
Waterplane properties:
Length on waterline : 20.745 [ft]
Beam on waterline : 6.417 [ft]
Waterplane area : 97.808 [ft2]
Waterplane coefficient : 0.7347
Waterplane center of floatation : 9.125 [ft]
Entrance angle : 18.366 [degr.]
Transverse moment of inertia : 246.63 [ft4]
Longitudinal moment of inertia : 2382.3 [ft4]
Initial stability:
Transverse metacentric height : 4.865 [ft]
Longitudinal metacentric height : 39.396 [ft]
Lateral plane:
Lateral area : 21.337 [ft2]
Longitudinal center of gravity : 11.163 [ft]
Vertical center of gravity : 0.706 [ft]

So once again any input would be much appreciated, and the use and speed requirments are still light use at around 15 knts

[Homefront]

Wow, that's pretty impressive stuff. I don't understand half of it but it sure looks cool. I'd say that with some back and forth here you'll end up with something buildable.
To my eye, I agree with tom regarding the stern width and will add that the house looks a bit tall.
Maybe when you're all done you could provide offsets to the forum.

[gonzo]

Lots on numbers, but the stern is really narrow. That looks like a sailboat hull. If you want to build a design like that, it is a waste of money to put a 75HP on it. 10HP will get you to hull speed with some reserve power. Any more and she will just squat.

[apex1]

Originally Posted by gonzo

Lots on numbers, but the stern is really narrow. That looks like a sailboat hull. If you want to build a design like that, it is a waste of money to put a 75HP on it. 10HP will get you to hull speed with some reserve power. Any more and she will just squat.

Gonzo our new designer wants a semiplaning boat, I doubt he will achieve that with 10hp. (not sure with 75 either)..............

[Ad Hoc]

Hmmmm...you say:
You have an L/B of 2.88..very low
your hp/ton = 29..very low, that's with the 75hp eng.

It'll make lots of waves....but not much else. Will look good as she does it though

as for:
"..Here are the hydrodynamics for her as is .."
These are terms called hull parameters. Hydrodynamics is the science of fluids in naval architecture, not hull parameters.

[JLIMA]

So basically if i were to widen the transom to say 80% of the BOA then what would you recommend for power, although I've seen similar arrangements on some of the local quahog boats and they seem to move pretty good. Although i need to find a pic to show you what I'm talking about.

[gonzo]

The displacement seems excesive. My Coronado25 sailboat was 4100lbs with 2100 of that in the ballast. You should be able to build it at half the weight or so.

[JLIMA]

I agree with the displacement seeming excessive, the last boat of similar size i owned was only 3,100 lbs and was steel hulled. Considering I'm planning on using plywood it seems rather extreme, but that is why I'm here to get other peoples input and help tweek the boat BEFORE i build it. If i build her as is would her sitting high like her actual material weight would suggest (only 1,800 lbs aprx.) would that have too extreme effect on stability or should i just except diplacement speeds and go with a much smaller engine, it would after all use less fuel. I do also have a 25 hp high thrust mariner i could put on her. In your opinion what would be easier adding ballast to get her neer her marks or adjusting displacement?

[apex1]

Originally Posted by JLIMA

I agree with the displacement seeming excessive, the last boat of similar size i owned was only 3,100 lbs and was steel hulled. Considering I'm planning on using plywood it seems rather extreme, but that is why I'm here to get other peoples input and help tweek the boat BEFORE i build it. If i build her as is would her sitting high like her actual material weight would suggest (only 1,800 lbs aprx.) would that have too extreme effect on stability or should i just except diplacement speeds and go with a much smaller engine, it would after all use less fuel. I do also have a 25 hp high thrust mariner i could put on her. In your opinion what would be easier adding ballast to get her neer her marks or adjusting displacement?

I would strongly recommend to get a designer / NA involved or to buy plans!

Regards
Richard

[gonzo]

Adding ballast to get the boat on her lines is really crude. It means that the design is faulty. Minor adjustments are OK, but you are talking 40% of the displacement in ballast.

[tom28571]

It's certainly possible to build that boat at a hull only weight of 1800# but the deadrise and lack of aft buoyancy shown would make it tender. Deadrise and displacement are related. You apparently have a computer program that is able to crank out all kinds of ratios and other data that, without some reasoned interpretation, is not worth much. It would take me several days to a week to generate all that data on a prospective design. I'm envious of those who can do that in a few minutes but am firmly attached to my pencil and calulator.

I would advise coming at this project from a different angle and working on understanding some basics of boat design fundamentals. Only a few such fundamental characteristics are needed to design a good boat for a first go at it. I have not tried to make my way through all the data and have never needed a large fraction of this stuff to produce an acceptably good performing boat.

Adding fixed ballast to a powerboat is a sure sign of a poor design.

[JLIMA]

Ok, so after decreasing the dead rise i was able to drop the displacement down to 2,600# and i also took the advice on widening the transom and I don't think it hurt the appearance of the boat, actually i think it's an improvement aesthetically. But there are a few months still before i start anything.