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. :cool:

[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]

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]

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.

[Homefront]

Looks better. :)

I'd sweeten the shear, and tweak the house, but you're off to a good start.

Maybe you could build a scale model?

[gonzo]

I think that the fallacy is in designing a hull with a waterline and expecting the boat to float on that line. The volume of water displaced has to equal the weight of the materials in the boat. If you need to add a lot of ballast to get it to its lines, either the beam is to great or the depth too much.

[JLIMA]

I don't want her to sit on her water line i want her to sit a little higher so there is room to load her with all my crap not to mention the effects of adding 900# of quahogs as i will undoubtedly be doing.

[apex1]

The need for a "proper" design or designer you still do´nt see?

[gonzo]

Then you need to desing a light and loaded waterlines. When there is such a difference you will need to figure out stability for both conditions.

[Easy Rider]

JLIMA,

I think the first design was OK but 20 to 25 hp and FD speeds of 6.5 to 7 knots would be the result. Batteries fuel and other heavy gear kept amidships would make her a balanced boat. The Albin 25 would be a very comparable boat except for the very soft chines. Go to atkinboatplans.com. Tang and Hope are excellent V bottom designs (especially Tang) and Wader, Marigold and Little Effort are flat bottomed. Both John and William Atkin are know for their excellent flat bottomed boats.

Easy Rider

[Mild Bill]

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.Will these pics do?

http://dngoodchild.com/7066.htm

[Homefront]

Guys, the man wants to do this himself.
I'm suggesting he build a large scale model to actually put in the water and test. He obviously has an imagination and an open mind to try this.

[JLIMA]

Thanks Bill those are more or less what i was talking about, but the exact ones there's 6 of them are across the harbor in Fairhaven. I just need to get off my ass and go take a pic of them. Those are pretty close though and are probably as close as can be found on the web.
About the scale model I intended to do a scale model before i build anything ... I always do one before i build anything other than a piece of furniture. Also this isn't the first boat I've built, just the first one i designed.

[frank smith]

There are a ton of stock designs that would fit the bill , can be bought cheaply and save you alot of aggravation . If you want to design it yourself , you can do that to , but I think you have a long way to go .

It might help if you went to all the common stock designer sites , Like Glen L,
clark craft , ect . They are all listed by our host . Then find all the models you like and make a list with all the specs for each boat . Take your time
with it . Now go back to freeships with that info and compare it to what you have drawn . Next make many drawings on freeship all different but similar ,
20 Iterations would be a good start . Also go tho boat yards and measure boats similar to the one you want . Soon you will start to have a feel for it .
Dont forget to stop by the library and get every boat book the have and read them . That is just my 2cents . Have fun .

Frank

[JLIMA]

Well, frank i thank you for your input, but I've already done alot of what you suggested. I was the most intrigued by the Jiffy V 22, and the C-Dorys, I'm a marine mechanic over at the Fairhaven Ship Yard. Having built 3 boats before it's just something i want to try and see how i do. I actually am currently farting around at another idea as well after i fiddle it a little more i'll post it here.

[frank smith]

Oh , bthw , 900 lb of clams is no big deal with the boat you want .
You show a hard chine boat so I m thinking plywood , say about 7'6" beam at water line for a start reducing to 7' or so at the stern . Deadrize at stern 10-12
degrees increasing to say 18 the bow .

[frank smith]

I like the jiffy V22 , very well thought out plan .

[JLIMA]

Here is the other idea I'm also considering but i haven't gone into any real detail on it yet. She's just a simple boat with a small pilot house and little else just something to get out of the weather. Low for easy boarding from a small boat and hauling my pots, with lots of deck aft of the house. But like I said it's just an alternative idea I'm pondering.

[JLIMA]

I actually just brought a Jiffyv in from Cuttyhunk around June, they handle wonderful

[tom28571]

Before I say anything about your latest sketch, would you offer some rationale for the bottom hull form? Did you just lay down some lines on the CAD or is there some operational logic behind the form?

[JLIMA]

Tom about the hull form, my logic was to keep the hull shallow aft so as i can use my short shaft outboard, I kept the bow a good deal on the fine side because the bay can toss a steep chop and i was trying to smooth the ride, not to mention I kinda have a thing for the lines of the old time Maine lobster boats. The added ease of planking in ply wood dictated that she be a chined hull. I use a mooring and have a bum leg which makes boarding a higher sided vessel painful from a small dingy (I'm too cheap to pay for a slip). I'm not in a hurry to go anywhere so low planing speed or even high displacement is fine for me, as long as I have room to put all my crap and I do carry a lot of crap. The last 2 influences were ease of build and the space i have to build in. Just wondering if there was a reason you asked (like you thought there could be an issue with the design) or are you just curious to my thought process? Either way it's a good question.

[frank smith]

I can get no idea what is goin on from that picture , how about a screen shot of the lines plan , or just all fews in freeships

[JLIMA]

That is just an idea sketch i haven't finished the lines plan for it yet but as soon asIi'm done they'll be up.

[JLIMA]

So here's the basic lines plan for her not a complete idea yet just something I was considering for a while figured i might as well post it too, and as of right now not the idea i want to go with although it was suggested i might try to do a flat bottom I would be concerned with pounding so thats not an idea I'm entertaining at the moment. But i haven't made up my mind as to what idea to go with.

[tom28571]

Ok, here are a couple thoughts about the latest sketch. A fine bow and a wide flat stern can be a problem and can even be quite dangerous. It is a receipt for boaching when running downwind in waves. The boat surfs into the back of the wave ahead and digs in. The following wave (if even slightly off angle) will want to slew the stern sideways and because there is nothing to stop it, the result can be a broach or even capsize. This is probably the cause of most serious operational mishaps in powerboats. Lobster boats have huge keel/skegs in an attempt to prevent this and are not all that fine forward anyway.

When quartering into waves upwind, the fine bow will enter a wave, which wants to push the bow aside. With no hull appendages to resist this, the result is a sharp lurch sideways. It's all too common and I owned a boat like this before learning that it was a terrible design. It was actually necessary to hold on tight to avoid being thrown sideways in these conditions.

You need more buoyancy forward and more directional stability aft. It's a compromise, but you need to know what the results are of compromising in either direction.

[apex1]

Is this going to be a "how to become a NA in a few hours" ? If so, may I subsrcibe also?
And is it possible to step in at the next level? Cos I know already how to play with Freeship.

[DBM]

Your design is starting to resemble a 50's era ChrisCraft Sportsman with a mild v in the bottom - which is not a bad design.

[Easy Rider]

JLIMA,

I really like Tom's comments and I'd like him to critique a design of my own but I don't know how to present it here. I'll learn and present it later.
About yours I;d like to add a point. Your boat's a planing hull and planing hulls ride on their bottoms amidships or slightly forward and at that point your bottom is taking the seas and where it's quite flat and the forefoot is out of the water doing no good at all. I thing we can all learn a great deal talking and studying other people's designs. I offer "atkinboatplans.com" as an excellent place to study.

Easy Rider

[tom28571]

Is this going to be a "how to become a NA in a few hours" ? If so, may I subsrcibe also?
And is it possible to step in at the next level? Cos I know already how to play with Freeship.

You're way ahead of me. I don't know how to run any computer design programs.

[apex1]

You're way ahead of me. I don't know how to run any computer design programs.

Oh there we can produce relief:

http://www.boatdesign.net/forums/design-software/step-step-design-your-boat-27900.html

enjoy.....:D

[frank smith]

There was an article in an edition of Woodenboat that was about three people who designed boats by eye .Very impressive .

[JLIMA]

I had some one pm me saying i should consider a flat bottom and they recommended a sort of a dory with a widened transom. I have concerns that with the chop that usually develops it will pound excessively. Is there a way around excessive pounding or would it just be more prudent to just finish tweaking the idea i have already?

[Oftengoofenoff]

abds
l

[Easy Rider]

JLIMA,

More deadrise, longer and narrower, soft chines, weight, flatter running attitude, extremely narrow multihull and longer WLL will help. Running a V bottomed boat into head seas at 20 to 30 degrees may present a relatively flat part of the bottom to the seas causing pounding that may be less w a flat bottom. Sound seems to amplify the effects of pounding and could intensify the experience so a quiet running boat may seem to pound less. When I designed a boat to address this issue I chose to use deadrise to solve the problem and I sorta went multihull to do it. The entire bottom of the "Easy Rider" had a deadrise of 45 degrees. Ray Hunt used deadrise to solve the same problem and countless other boat designers since. One may even use suspended seats like trucks to lessen the effects. I know of no solutions that don't offer significant penalties that offer significant benefits to the problem of pounding.

Easy Rider

[Oftengoofenoff]

sorry about that reply my wifes damned cat ran across my keyboard. You still might want to look at the freeship file i sent you... at least i assume your using freeship.

[JLIMA]

Well I'm off, starting the build, will post pics, and keep ya'll informed

[lewisboats]

OK...what are your thoughts on the various thicknesses of the planking of the hull and the cabin and superstructure? What are weights and lever arms? What is the total CG and how are are you getting there? Have you placed the engine, the internals and the overall CG and is the design balanced?

[PAR]

You're starting to sound like me Steve . . .

[robbie2161]

Hi
Sorry to just go back to the design process as I know you’ve started building. I just had a few questions with regard to the comments and the designs (which I like the look of!)

Please correct me if I make a mistake here as I’m limited on time.

Looking at the first design and this question is really for Tom

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.

I agree this applies for the majority of planeing vessels, but in the case of a transitional vessel (or semi planeing). Wouldn’t you get away with a narrower transom in a transitional state? To me as long heavy items such as batteries and fuel tanks are placed intelligently low down and not too far back the weight acting further forward on the longitudinal centre of gravity will soften the ride and allow you to get away will a transom such as the designed one providing a more comfortable boat in small sharp chop and better wave surfing characteristics.

Ad hoc

Which hull design are you talking about the first or the second as I got a L/B of 3.07 Taking the overall length and beam? This is in the beamier range of a planeing boat. 2.88 would be a very wide boat and far from transitional. Is this what you meant by ‘very small’?

Both the vessels have a Froude number of 0.72 and 0.76 calculated with at a speed of 12kts and speed/lwl of 0.64 and 0.79. with regards to Fn and the speed/LWL this is on borderline planeing.

10 kts 0.61 and 0.64 and the speed/lwl 0.37 and 0.48
With regards to the Fn this is transitional

There are different opinions as to what the boundaries between the three types (transitional, planeing and Disp). The way to calculate it is Fn

Generally speaking if the Fn is 0.4 or less, it is classed as a displacement craft.
0.4 to 0.82 is the semi displacement category and 0.82 and above is classed as planeing. There are exceptions for example if you over power a displacement vessel it is possible to get it to go beyond the 0.4 boundary but at the expense of fuel efficiency. (Norman L Skene. ‘elements of yacht Design’ Seafarer books)

Also another question is the powering aspect. Establishing that we are in the transitional area and taking into account 2.5 and 1.7 tonnes as a displacements which I have to agree is heavy for a boat of that size but it’s your boat fill it with what you want.

Even with these high stated displacements this gives the first designed vessel a hp/disp of 41.39 and the second one a hp/disp of 53.53. Again I find this is well into the transitional category.

With regards to excessive pounding it is the position of the people on board as well as the hull design. I did a bit of research into hull forms by installing accelerometers, from the research it is said that that 85% of the overall length from the bow is the position of minimum of vertical movement so quite far back.

Designing a comfortable seakeeping hull form seems to be an art rather than a science at first but it can be done scientifically. I have a literature review I did on the subject of seakeeping if you would like me to pass it on, it only has about 10 to 15 documents as there isn’t much in the way of information in that area, hence my research.

And finally what parameters did you decide upon for your hull? Interested to know great project, keep up the good work!

Rob