Not wanting to over power it.

[Craig87]

Hello a couple of months ago i bought a 6 meter boat to restore . It had no engine and all the wood was rotten its striped out now and fiberglass repairs on the crack cabin roof are underway and all the hard by hand sanding is done (thank god)
I am not not 100% sure of the engine to put in it . I like the volvo penta engines as parts are easy to come buy and they seem to be pretty reliable motors. After some calculations hull speed will be just under 6 knots and it would take 7.8hp to get it there witch would leave the 2001 engine with 9hp my first choice but I was wondering if a 18hp 2002 would be ok in it . The reason for wanting more power is not for more speed more due to the tidal l moorings it will be on and the fact that i am amusing a second hand 2001 rated new at 9 hp is not going to be putting that out after 4000+ hours. So any insight into this would be great as the boat had no plate with the ratings on so i am at a bit of a wall with it and am planing on getting the engine in the next month or two . If it would be at all un safe to over power it i think i would stick with the 9hp
Thanks
Craig

 

[Nautorius]

Craig,

It depends on the type of hull, beam of the boat, weight etc. Are we talking Inboard such as Volvo Penta or outboard engine. Also is this a displacement or planning boat? If it is displacement then it does not matter if you over power it as it can only go at Max hull speed (it will be just a waste of Money) Have you a picture of the boat maybe?

Cheers

Paul

 

[Craig87]

Hi Nautoriuse thanks for the reply it is a displacement hull and will be powered by a inboard motor its hull speed i have worked out is just under 6 knots and would take 8.7hp to power it to that . I am thinking about some thing more powerful as the single cylinders i have looked at seem to be the complete opposite of smooth and i don't want to be shaken to bits . All so i am worried that a old second hand 2001 would not produce the 9hp it says from new so might end up being underpowered.but at the same time i have been told to much power can be dangerous and am worried 18hp might not be safe i would be safe I know it will never go faster than hull speed but if it was safe and could be run at 3/4 throtle it would make for a smother and quieter ride i would have thought but truly have no idea
Thanks
Craig

 

[Chris_d]

No you are not overpowering, yes a bit more than required but no where near excessive. Of course weight and size and the correct gearbox ratio and prop size will be crucial, I wouldn't worry too much about the hp. What you need to calculate is a what rpm the larger engine will be producing the 9hp you need and get the prop and gearbox setup to match.

 

[Craig87]

Excellent Thanks Chris_d . if i am stuck with a set gearbox could I have the prop made to mach as i am looking at second hand and many of them come with the 2.4-1 ratio. I have entered the information into a prop calculator online and has given me detentions but i assume i need to look at the power curve and work out the right rpm and enter again? Not sure if that would be ok or not . Sorry this is all new to me and the learning curve is HUGE lol
Thanks
Craig

 

[Craig87]

Im also not 100% sure what the weight will be exactly and would be going on a bit of maths and if im honest 60% guess . She is 20ft long and 7 wide . Im am asuming once wood and engine is all added it would be around 2ton would that be way out the ball park and any idea how i can work out the wight ? as where it is i have no idea how i could.
Thanks again