How do Boats get there HP ratings ?

[john_m]

Hi

I have been looking at the Seajay escape 4.55 and this boat is rated max 50hp at 115 kg for motor (tiller)
weight wise i could put on the 60hp yammie 4st but it is over rated in HP.
But
the seajay escape 4.5 sports 4.55 is rated max 60hp at 120 kg for motor (non-tiller)
the only difference in the sports is that it has a side console.

I wouldn't think they would make many mods to a hull apart from bolting on a console.

Do non-tiller allow more weight as the boat would be more balanced to the front
then tiller where the weight is further back?

I can only think of how they get there ratings are

strength/stress on the transom.

Weight on transom so it won,t sink

Thanks
John

[2iar]

I don't think you'll ever get a straight answer on this one. I certainly didn't when I contacted MSQ.

However, in addition to what you've said, you'd have to also include the centre of gravity of the boat itself. For example a boat with a cabin has it's COG a bit higher than say, a ski boat. Therefore it's "safer" to perform a tight turn at speed in the ski boat (all other things being equal) than the cabin, while having the same HP on the back. That is, you might be able to have a larger HP on the ski boat that could be dangerous on the cabin boat.

I realise that this doesn't explain why the side console should be any different - indeed logically, a side console has a more uneven weight distribution. Just pointing out that there may be more factors than transom "strength".

Give MSQ a call or an email. They're obliged to answer the query, even if it doesn't particularly help.

Good luck,
Mike

[john_laura]

Hi

I would say that it all comes from when the boat is designed and the engineering calculations that are worked out with the strength of thr transom.

Once the designs are worked out it could be possible that some manufactuers err on the side of caution in there ratings(possibly insurer enforced) and others use there higher ratings as a sales pitch.

Also on my 4.5m tillar steer it is limited to 50hp. I have found this to be more than enough and it can be rather difficult to see before I'm on the plane.

John

[Fishin_Dan]

Would this be more due to the fact of the weight than anything else... (The difference between the 2 that is)

In a tiller steer, you are at the transom and so is the motor. Therefor you would have the extra weight (Say 70-100kgs) of the driver right at the back. This will lift the nose, and push down the back more.

In the console (Centre or Side), you are half way up the boat approximately, so the extra weight at the back will not lift the nose anywhere near as much.


Overall though, if you put too much HP on the back, the transom will give out in one way or another...


This is just my thoughts anyway

[Archer]

Just on this note..well sort of lol
Ive recently picked up a 3m punt. It has no ratings plate on it at all. It currently has a 3Hp on the back and i am thinking of upgrading that to somthing with a bit more stick. Whats the go with not knowing the Max HP rating? Do i need to get it looked at by someone to tell me this or can i just wack a 10-15Hp on the back and away i go? :-/

[2iar]

It's a curly one mate. You could try getting in touch with the manufacturer if you know the exact make/model and year of manufacture. If you can't get it from them, I'd say that you're then responsible for taking due care in what motor you fit.

As for the 10/15 rating, it might be of interest to you that each brand normally weighs the same for both models. Quite often the 10 is actually more expensive then 15. She'll fly with a 15 I reckon, but I'd strongly suggest talking to MSQ before fitting one, and if possible get something in writing, which might then help with insurance...

Good luck,
Mike

[Jeremy]

This is a good question John. It is the boat manufacturer who sets the hp rating. If you want more horses than what it is rated to, I would suggest taking your boat back to the manufacturer and asking them nicely if they would re-rate your hull. Stessl did this for me (35 re-rated to 40 hp), and the cost was $100 for a new compliance plate.

Archer, the main problem I can see if you simply take a guess and get it wrong is insurance. Both for your boat and the damage it may cause if excess hp was deemed to be the cause of an accident. If you want to be safe, go to a marine engineer (as a guess) and ask them to give it a hp rating. There would be a fee as they would have to cover their arse as well as their time.

Jeremy

[Archer]

Ok thanks guys, Ive emailed Ally craft with all the details i can find on the boat. Hopefully they can sort it out for me.
Perhaps 15 would be too much for the small punt?? Im not sure all i know is 3Hp is not enough. :o

Cheers Joe

[2iar]

It might be Joe, but you'd have a few options - 4,6,8,9.9, 15. Maybe have a look at the Tohatsu 9.8?

I just wanted to point out that it's not always good economy to buy a 9.9 over a 15 if it weighs the same and the 15 is cheaper. If you used the 15 at 50% throttle, and the 9.9 closer to WOT, you'd find you're fuel economy to be better, and you won't be flogging your motor.

It would be interesting to see if you get an answer ot of Ally Craft.

Good luck,
Mike

[FNQCairns]

Weight is where it is at, the HP factor (within reason) is largely to account for the lowest common denominator of driver or for marketing purposes to force a boat to compete for sales in a particular area.

Archer if your boat is a true punt 6hp will push it fine and keep you safe, 15hp could see it easily flipped around a river bend into the wind (I know ;)), as mentioned above a 15 can be driven to only ever put out 6hp if so inclined and some are only as heavy as a 9.9.

cheers fnq

[Archer]

Thanks again guys, Not sure whether Ally craft will be bothered to reply about a 15 year old boat but one can only try...If no reply to the email ill give them a call see if that makes any diffrence. The boat should have some sort of compliance plate though yes?
FNQ, Yea its a punt. Im not after a racing boat just something to crusie around the rivers and small creeks flicking SP's in. Sure dont want to go flipping it thats for sure!! ;)
John, Sorry for hijacking your thread. Maybe one of the MOD's wants to move this to its own thread?

Joe

[TonyM]

John - I reckon the comments above have summed it up quite well. Jeremys suggestion to approach the manufacturer sounds a great idea, certainly wouldn't hurt to try.

Archer - Personally I'd be looking at somewhere between 6 and 9/10 hp for a boat that size.

Normally for a smaller boat like that I never worry about insurance but that's a personal thing - if you want it insured you must ensure it complies with manufacturers specifications or your cover is not valid.

With a 6 on it it should get up on the plane without tooo much effort, with a 9 or above it should have a nip in it's tail ;)

[Reef_Ranger]

Joe, I have a 3.40m Qunitrex Traveller with a 9.8 Tohatsu. #Great combination. #Tough motor, always starts and has plenty of go, and only 27kgs :). #Easily planes with two adults and three little kids. #

Years ago I had a Savage Kingfisher 10ft with a 15hp Evinrude - This motor was a little heavy, but the power was great to have up your sleave, especially in the floodwaters of western Qld.

Sam

[john_m]

Thanks for all your responses guys even if we did get a little off track
but it,s great to get 2 tings sorted in one thread.


I think i will e-mail Seajay and see what there response is

I still don,t get the Weight argument as the boat is rated 115kg and 50hp
i could either put on yammie50hp 2st at 93kg
or put on yammie60hp 4st at 114kg
and i am still under weight

it just come to me might also be thrust on transom also.
anyway i will find out from the manufacturer and get back with answers

[john_m]

Okay
Well i called the manufacturer and they said that the sports has steering thats why it can take a 60 hp.....

I also asked how that get there figures and was told the following

They must comply to a formula in the australian standards.
Which with further investigation it is the AS 1799 or thats parts of it .
i need to purchase that to look at it so i don,t know the formula.
or how it,s worked out....

So to any one who didn,t know here is the explanation

[bushbeachboy]

John,
Before spending any money, look for some back issues of F & B Magazine. They had an article in there in the last year or so with the formula and its applications included. Not sure about the author of the article, but it was in the back of the mag and by a marine engineer. (I think) Sounded authoritative anyway.
Cheers