New boat...New boater

[keana]

Can anyone answer this question for me please. It might be a bit long winded but i`d appreciate some help here as i`m new to boating.
I`ve bought a 15 ft Bonwitco boat (semi-planing hull) with a (1983) 25hp Merc 2 stroke engine. Anyway, it came as a bit of a bargain as the bloke I bought it from hadnt used it for 12 years and it was stuck on his drive all that time. All I did was replace the starter motor and it started first time.
I`ve spent a bit on it since with GPS, DSC radio, fishfinder etc. The first time I went out was on my own out of Penarth into the Bristol Channel. Pottered around the coast getting used to it etc. Brilliant....22knots up on the plane straight away. The next time I went was fishing with my son. The most I could get flat out of it was 11 knots. I got him to move up into the bow and it improved by a knot or two. He`s a big lad but does the weight of one person really have that effect on performance? It appeared that the stern was really digging in so I altered the trim of the engine by moving it a couple of holes. That didnt seem to make much difference either. Someone told me to buy some trim tabs and others have told me not to bother as they are for bigger boats. I also seem to be using a lot of fuel (about 2 galls an hour) which doesnt seem right.

The question is.....should I buy some trim tabs or would proper weight distribution suffice?

Brill if someone can help with this one.

[big shaner]

hello mate i too have my own boat an orkney 16ft fast liner and i wouldnt have any other boat its got a 15hp yamaha on the back you have to remember when boating the diffrent tides size of tides etc etc so that might explain a few of your probs did you ask the person you bought the boat off if it was flushed regualy with fresh water when he had i know sometimes a salt build up can alter the performance of the boat and also have you got the right oil petrol mixture right

[Lookfar]

Hi,
Assuming the only difference between the two trips was the addition of your son, I'd say the extra weight is preventing the boat getting out of the hole. To plane, the boat has to climb over its own bow-wave and get the front-end clear. Even 2-up a 25hp engine ought to be enough but if yours is running a bit rough or your hull is particularly inefficient then the boat will just bog down at about the speed it'd normally start to plane at. 11 knots sounds right for that.
For 2 stroke fuel consumption on a 25hp 15 footer, expect 4-6mpg as a ball-park figure. If you were doing 11 knots then 2 gallons per hour is about right.

Steve

[duncan]

I would echo Steve's observation on fuel. New out the box your engine would be using 2.6 gallons ph at full throttle. With 2 people on board you will not really get a return for your money at high thottle settings - better to run at displacement speeds and low throttle settings - ie you will probably get 5 knots for 0.6 gph so much better mpg. 1 up 14 knots will probably be a good balance between covering ground and mpg.

However I am more of the view that it is simply power/weight - yes the is a large resistance hump but at 11 knots a 15ft hull is already through most of that. Especially an SP/SD hull shape.

Basically everything you have outlined sounds exactly what I would predict!

Don't bother with trim tabs

Don't get tempted with Doel fins on the OB either

Do keep the trim on the engine set correctly for one person - then use the second as you have been doing as movable trim.

Enjoy your boating

[keana]

Thanks for all your replies fellas. It sounds like most things are happening as they should be then. It all sounds very much more technical than I realised it would be. It seems as though weight distribution is the key to it plus a little bit more ooomphhh to get her over her own bow wave. Maybe its a case of getting to know your boat and its capabilities as well.............I`m impressed and indepted to you all.

[nimue]

Hi,
Yes one person makes a huge difference when power is limited, Don't know how your funds are but i would go for a bigger engine if your son will be going out with you quite often. On seriously tidal areas like the Bristol channel you are very limited on the times you can launch and recover so speed can be essential to safety if you want to cover decent fishing grounds.
Be safe
Jules

[Greenheart]

Hi Keana, don't know much about boat engines, classic motorcycles are more my thing. But if an engine has been stood for 12 years I would definately check the fuel system. Modern unleaded petrol has a tendancy to gum up & block the carb jets etc if not run for a bit. Check the obvious & simple things first like fuel filters then consider having the carb apart.
Mind you my old Triumph Bonneville slows down a fair bit when the missus is on the pillion!!!

[keana]

Thanks folks. I took her (the boat that is) out yesterday. What a difference a bit of weight in the bow makes. I flattened some old lead pipe and shoved that up in the bow. Got my mate to sit up there too. We were doing 17/18 knots in no time and against the tide and wind too. Nice flat ride into quite a choppy sea. Fuel consumption seemed to improve as well because presumably we were on top of the waves instead of trying to plough through them.
Didnt catch any fish though....but hey..cant have everything.