max hp for boat

[Deiter]

I have an 5.5m platey (Profish) without a manufacturers plate to give max hp / transom weight.

Now i seem to remember a little while ago that there was some sort of calculation for instances such as this to calculate the max size for a given boat.

It went something like, length x weight / beam + todays date - no. of spanner in a sidchrome toolkit blah blah blah.

any one know what the right calculation is or did i imagine it?

cheers,
Damo

[davez104]

I don't think there would be a formula for working out max horspower. There may be one for working out how many horses you should have though. Max horse/weight rating would depend on lots of things i would think, V angle, beam, weight, boyancy in the back end, how strongly built the boat is etc etc. i would try calling profish and ask them, they may be able to replate it for you.

Dave.

[Deiter]

cheers dave, but i did that already. Problem is, my boat was built before the company was sold to the current owner, and he wasn't sure. he gave me roundabouts figures, but was giving me numbers based on his current hulls in my size.

He also said that he has changed a few things since then, including a shallower vee and slightly narrower beam.

The thing is, if there is no plate specifying my max weight/hp, how do i go come time to make an insurance claim??? I wondered if they would use some formula for hulls not stamped?

Damo

[blaze]

there is a formula to work out max hp for boats, cant remember where it is but its the lenth of the boat x the width at the WATER LINE. I will have a hunt for it. It may suprise some people how small there max can be.
cheers
blaze

[Deiter]

Thanx Blaze, you are a champion.

waiting in anticipation....

[davez104]

HMMM, seems a bit dodgey that theres a simple formula for working out max HP. Surely construction material, thickness of hull and all that sort of thing should be taken into acount too? Can insurance companies use this magic formula to override the manufacturers specs? I would doubt it but I reckon there would be heaps of plated boats out there that won't fit into the formula. Very interested to see the formula and apply it to a few boats.

Dave.

[Deiter]

Point taken dave, but it does exist.
for what express purpose, i am unsure as yet, but we may find out.

Not sure on the insurance side of things either, hence the queery.

[davez104]

Hey, I don't disbelieve that it exsists, if that makes sence. Just sounds odd, as they say, boats aint boats. Also very interested to see what formula is used and how the results compare to what is acctually plated on a few boats.

Dave. :)

[snapperm8]

well its not just the length x width u need to consider the wieght obviously and even the hull structure but the more the merryer ;D

cheers dave

[Deiter]

he he ;D, w/ any luck i will get a max hp of 140 and a weight of 190kg. Then i can bug Kopey :D

[foggy]

Found this in an old post whilst searching for something else.


"Australian Standards Calculation AS 1799
Length of boat X Waterline beam width at transom X 16 less 67 then X 1.34 = hp you can use on your boat.
So an example might be
6m boat X 2.14m =12.84 X 16 =205.44 less 67= 138.44 X1.34 =185.5 hp
A serious practical on water test has also to be done, at full throttle, through an on water course,performing a 90 degree right angle turn, in a prescribed distance.
Sounds scarey to me, for some overpowered boats."

Foggy

[blaze]

thanks foggy, thats the one
cheers
blaze

[marshy]

Hello all, been reading with interest.by applying Foggys formula to my boat it comes out at 63.82 hp, it has a70 but is plated for 115. Length is4.55 beam at waterline on transom is1.61 if anyone want to check my calculations [maths arn't my strongpoint] By the way 70 seams plenty of power for my boat. I wonder why manufacturers overstate the power ? is it to get more sales ?.

[Deiter]

Good work Foggy, that's the one i was after. Much appreciated.

Gives my boat a max of 146hp.

Marshy, your calculations are close, i got 67hp for your boat. 115hp max???

Is it made in the U.S???

Now, does anyone know which, if any, authorities might use this calc? And for what purpose?

so many questions i know, but it is interesting.

Damo

[marshy]

yeh damo, very interesting. I was just thinking about some of the ski boats you see, by the formula they would have about 3 times more than max recommended power. By the way you must be pleased, now you can get that 140.

Marshy :)

[Deiter]

most :)

[davez104]

According to that my 3.95 barra punt can take a max of 45HP, its only plated to 25HP but I have run it with a 50 2 stroke, I don't feel so bad now. But my 5.2M Haines should have a max of 120HP, not sure, but I think they where rated to 160 or thereabouts. Mine runs a 135 and she's sweet. I guess my point is, whats the relevence of this formula? Who uses it and for what purpose? It's obvious that boat building companies don't use it, and nor should they IMO. But the fact that it exists means that it must have a purpose. Can anybody enlighten us?

Thanks.
Dave.

[Feral]

Last part of test seems flawed, surely that would depend on maximum speed which once the boat is on the plane, is more related to how you have it propped? (the horsepower of course gives it the ability to push the prop to max revs, but once prop reaches max revs, additional horsepower is of no use)

You could put an enormous motor on, and a thin as crap prop and only get a slow speed for the test allowing the turn test to be easily passed?.

I would reckon the formula exists for calculations for boats not covered by manufacturers specifications, sort of like the law on towing trailers, (ie if you car does not have a manufacturers specified maximum towing weight it is the same weight as the vehicle).

Manufacturers of course have engineering design calcs done based on the transom and hull configuration, and these calcs set the maximum horsepower and weight the transom can take.

[Sportfish_5]

PM Gary Fooks

I think it was his post originally with the formula.

[oldyellajim]

so the heavier the motor you fit the further the boat sinks into the water, therefore the ggreater the transom waterline beam. This means more horsepower is allowed??? :-?

Jim

[Spaniard_King]

Just put a 150 on it an never stop 8-)

Garry

[FNQCairns]

IMO the calcs are used as an 'out' for whoever needs it also marketing to force a particular sizes boat into a range it wouldn't be in naturally, HP output is goverened by the fool behind the wheel, most important is weight when at rest once this is heavy enough to compromise the craft in the conditions it sees, it then becomes dangerous. Also weight when swamped has importance also. I have never felt bad being over HP it's only unsafe if used as unsafe. If a transom with the plated ability to handle say 100hp 2s cannot handle a 160hp 2s the manufacturer deserves to go broke fast.

cheers fnq

[Grand_Marlin]

I would reckon the formula exists for calculations for boats not covered by manufacturers specifications, sort of like the law on towing trailers, #(ie if you car does not have a manufacturers specified maximum towing weight it is the same weight as the vehicle).


I would think that this would be the case ... it isnt flawless by any means, but it does give a good starting reference.

If an incident were to happen where it was thought the boat was overpowered, the courts and insurance companies would call up the Australian Standard (amongst many other things) as a reference.

If your boat was within the guidelines, then you would have some chance of reprieve.

The obvious advantage is that it stops people recommending 150 4 stroke Hondas on 3m tinnies :D ;D

Cheers

Pete

[Sniper]

Just put whatever size you want on there. If it sinks, drop back in size till she stays above water. Insurance might not like it but. Have you called profish? How old is the boat? I thought all boats had to have a max hp rating on them, not that som people take any notice of them. lol
Bill

[plat-a-puss]

g,day guys have been reading with interest . in the comming future im going to build my own transom on a back of a formosa 4.5 ,have been told to contact a boat wright so they can do there measurements to determin rec h/p by doing this it will cover your but with the insurance people, so my suggestion would b to contact a boat wright to get there formula and then work out your rec h/p