Suitable engine prop hull combination

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Bosunof

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I have a 15 ton, 42 ft wooden boat with a 35horse Bukh engine. At the engine speed recommended, 2500rpm, I can only get 4.5 knts out of her. Clean hull and no wind ... 3 bladed prop fixed pitch. Shouldn't I be able to get more out of her than that?
Or should I get the log speed indicator calibrated?
Should I change my cast iron topsail for a modern one?
 
A true "auxilliary"! Today you would put 50+ hp to get near to hull speed. First test is to see if it will go up to max revs which I guess is about 3000. If it does that easily you may be able to coarsen the pitch a bit. You could find the size of prop usually stamped on the boss and then ask a propeller supplier for a recommendation - you will need WL, displacement, power and reduction ratio (for shaft speed). You can get on line calculators (Google Prop calculators) if you want to do the calculations yourself. If your prop is in an apeture behind a long keel it is likely to be inefficient. Larger diameter may help if your engine will drive it, but you may have clearance problems and it will reduce speed under sail.

Welcome to the world of boats that were designed to sail and not motor!
 
Sounds about right for a hefty boat to me. You might squeeze another half knot by fiddling with prop sizes. Nowadays you would expect to fit 50 -80hp in a hull that size.

Easy way to check if your log is roughly right - borrow or buy a handheld GPS. They nearly all have speed indication. Just make sure there is no current running when you check it. You may be quite surprised at the non-linearity of the log! I set my log to read accurately at around 5.5knots which is around my average speed under sail; log speed is lower than GPS at 2 knots, but agrees at 5kts
 
are you getting full engine revs?? or has it got more to go??, if will rev higher still, you're definately underpropped

just been down the same route, although we've got 120 horses to play with, for us we needed to get the shaft speed down by increasing the gearbox ration - we had a 1.5:1 box and 21" prop, we're going for a 3:1 box and 28" prop (45' boat)

with a slower shaft speed, and a bigger prop, you'll get more torque at the prop and a few knots more. hull speed for 40foot is going to be round 8knts
 
Thank you all. I shall have a go at sorting out what the hull speed is and the engine details when I am at the boat tomorrow. After all that is where I keep all the details. I do get 2600 rpm from a 35 hp Bukh. It gets up there after a moment or two and will not go beyond.

I shall report back what I find, if you would find that at all interesting.

It will be next week when I calibrate the log following a few runs with the GPS running. Hopefully I can get things sorted with the instruments... I shall use GPS and a Walker patent log so I should have a bit of fun.
 
The current Bukh 36 has a continuous rating rpm of 3600, so if you are only getting max 2600 either something is seriously wrong with your engine or you are over rather than under propped. Check when you run it at max throttle whether you get black smoke. This is the normal indicator of over propping because you are feeding the engine with more fuel than it can burn because the prop is holding the engine back.

4.5 knots at 2000 rpm where a max is well over 3000 is about par for the course. With the correct prop you should be able to get over 6 in flat water, but will lose in a head sea because by today's standards the boat is underpowered.

Hope this helps
 
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