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hacd
03-23-2001, 01:30 PM
im in the process of designing a 24 foot, outboard powered boat, and having more or less finalized the design want to test a scale model of the craft to validate the design before committing to building.
(the model is 6 foot long)

I dont have access to any facility that could test the model and intend to tow the thing along side a small boat in varing conditions .

does any body have any bright ideas (and inexpensive)
as to the best way to tow the thing ,so as to replicate the effects being pushed by an outboard???????????

Keith Wilson
03-23-2001, 02:21 PM
Find a copy of Weston Farmer's book "From My Old Boat Shop". Aside from being a good book, it has a detailed section on exactly that subject. The more recent second edition is better, has some additional material, but both have the bit about model testing.

Ian McColgin
03-23-2001, 05:22 PM
If I remember correctly, Farmer has the best remark about scales, equivalent speeds, weight scales etc. It's fun to "tank test" off another boat or walking along a pier.

Oh yeah. OB. Maybe bicycle along that pier . . . I don't know that Farmer's remarks go up to planing hulls.

G'luck

[This message has been edited by Ian McColgin (edited 03-23-2001).]

Tom Lathrop
03-23-2001, 09:37 PM
Hayseed,

I did what you contemplate a couple of years ago for a 24ft outboard powered planing hull except my model was 4ft long.

What characteristics are you interesting in studying? It's possible to get some useful information but data that is accurately translatable to full scale is apparently complicated to obtain. I looked at towing resistance and handling relative to different hull forms in the aft part of the hull bottom. The model was towed alongside a skiff well clear of wave action.

I didn't have Farmer's book at the time but got scraps of information from various sources. I plan to go further with the towing tests although the boat has already been launched.

[This message has been edited by Tom Lathrop (edited 03-23-2001).]

George Roberts
03-24-2001, 11:14 AM
Scale model testing is very difficult.

There are several nondimensional numbers that you need to keep constant. Reynolds number (surface drag), and Freud number (wave drag) are just 2.

Since you have only one variable (speed), results need to be carefully viewed.

hacd
03-24-2001, 11:34 AM
Originally posted by Tom Lathrop:
Hayseed,

I did what you contemplate a couple of years ago for a 24ft outboard powered planing hull except my model was 4ft long.

What characteristics are you interesting in studying? It's possible to get some useful information but data that is accurately translatable to full scale is apparently complicated to obtain. I looked at towing resistance and handling relative to different hull forms in the aft part of the hull bottom. The model was towed alongside a skiff well clear of wave action.

I didn't have Farmer's book at the time but got scraps of information from various sources. I plan to go further with the towing tests although the boat has already been launched.

[This message has been edited by Tom Lathrop (edited 03-23-2001).]

thanks for the advice im now searching for farmers book.

i built the model at six foot as i had some scraps of that cheap 1/8 indonesian ply lying around that size and figured that -

if you could bend the ply easily enough,it should follow that there should be no unforseen problems full size

in building 3 previuos boats from plans every single attempt has had some ply bending problems , ranging from poor quality ply to the desired shape being undelopable(nice to find out when every thing is buttered with $$$ worth of epoxyect)

so i dont want those problems again
designers should make an attempt to work outsuch elementary bugs from their designs before flogging them off to the public

this boat is a bit on the low hp side, and skinny , i need to find out if the aft bottom works , also the resistance, and stability,,and reaction to waves ect


i think i would find it easier to test an inboard with shaft model than an out board as theengine trim anlges and variuos centers of this that and the oth4r are more critical(for the out board)


i think it would be interesting to test your model retroactively, and see how it compares to the full size version

Tom Lathrop
03-24-2001, 07:32 PM
Other than towing resistance, the results were obtained by visual observation of the model with differing bottom configurations at various speeds, surface conditions and displacement. With these simple setups you have to limit what you expect to find out. Exhaustive results are out of the range of amateurs with limited budgets and facilities.

Low HP and skinny can make a good boat. Are looking at something similar to the Coyote displacement hull form (in Farmer's book) or a planing hull?

I got most of my design information from a borowed copy of Lindsay Lords "Naval Architecture of Planing Hulls" which was printed in the 40's. Copies are rare but should be found in some libraries. This is, by far, the best book on the subject that I've found.

I followed his recommendations and the boat performs really well.