I have a Hunter Horizon 26 with the factory fitted Yanmar 1GM10 (recently overhauled) & 2 blade prop. When the engine is required, the propulsion on offer is inadequate for reasonable progress. Will fitting a 3 blade prop provide a reasonable improvement without overwhelming the engine or creating undue tortional loads in the drive shaft?
Any advise on offer would be gratefully received.

Fitting the right prop to the engine is something of a black art. You need to know the desired pitch as well as the size of the prop. It might be worth investigating if the prop you currently have is the right size/pitch before you do anything about changing it. A three-bladed prop might give you more drive, but it would also adversely affect sailing performance, since it would create more drag when the engine was not in use. Theoretically a two-bladed prop should be fine on an easily driven hull like a Hunter Horizon. What speed are you currently motoring at, and what speed do you hope to achieve for "reasonable progress"?

This is a really complcated subject - I have been there several times myself and it is almost impossible to get a straightforward answer from anyone.
However .....
when you say you don't go very fast, what do you mean, you should be getting around 5.5 to 5.8 knots in flat water with the engine getting up near to its maximum revs, If it is reaching maximum revs at a lower speed, the prop is either too small or has not got enough pitch (NB props can be re-pitched cheaply - new props are very expensive!), If you are getting say around 5Kts and the engine is no where near its maximum revs, your prop is too big or overpitched.
If you find that all is OK in flat water but motoring into a head sea or strong wind you only go slowly, then again you need to look at the engine revs in these conditions, if the boat gets slower but the engine revs stay the same, then the prop is too small, if the boat gets slower and the engine revs fall away a lot, the prop may be too big, however 10hp is not going to do a lot for you against a force 7 or 8 anyway!
Fitting a 3 bladed prop on small engine tends not to do much for you as the benefits of the additional blade area are usually consumed by the additional frontal drag of the third blade.
One thing you really should look at is your bottom finish, a racing quality finish makes a collosal difference when motoring. Also you should look at the prop itself, is it fouled, has it got a really superb finish or is it rough and ready, this really does make a major difference.
The other down-side of a 3 bladed prop is that it will ruin your sailing performance.
You might want to consider a folding or feathering prop, not only do they improve your sailing performance (as much as 1/2 a knot upwind, but they are usually much better at motoring the boat as well! Don't believe the stories about them not working in reverse, this is simply an old wives tale, they are actually more efficient astern because the blades angle into the flow slightly, ask anyone with a folding prop and they'll tell you they can motor faster astern than ahead!

Thanks for this info.
In answer; The hull of my boat has just been stripped back to gel coat & re-antifouled with coppercoat, so I am happy with how slippery the bottom is through the water. I am more interested in sailing the boat than motoring it, so am not willing to compromise sailing performance with the third blade.
One issue from what you say is possibly me not revving the engine high enough, it has only just gone over the running in hours after the rebuild so I have been gentle with it. The other thing is that a muffler has not been fitted & she kits out a loud bark when opened up (which i don't like much) & am currently looking into rectifying this. I have struggled to get more than 4 knots from her in flat water but will get out in Southampton water & open her up to see how much of that is my lack of enthusiasm with the throttle. If I could get 4.5 - 5 knots (without defening the neighbours) I would be happy. I will also look at the throttle linkage, as it was replaced by the previous owner & may not be set up correctly to achieve full throttle opening.

Sounds like soundproofing and maybe looking at the throttle linkage could well solve your problems - a lot more cheaply than buying a new prop. Let us know how you get on! I have often been told that diesel engines thrive best on being worked hard. So there is no need to be afraid of using more revs - as long as you arrange things so that your ears can stand it!

I have now fitted a Vetus NLP45 waterlock to my exhaust & what a difference! This unit does a brilliant job of quietening things down while not taking much space up on the boat (not cheap though)
I have had the engine running at full throttle (not altered yet) & have only managed 3.8 knots in slack water. Also when running against the tide, the engine does not seem to labour at all. The engine sounds to be revving at as good rate & the increase gained at various revs leads me to believe that it wont get up to 5 knots plus even if it is revved hard.
The next step, I assume, will be to get the prop pitch checked when lifted out. Do engineers capable of setting prop pitch have access to the data required or is it a trial & error job?

Is there a Horizon owners' association which can supply information about the correct size and pitch of the prop as originally fitted?
Is it possible that the problem is not the prop but the gear box?
We had the equivalent of "clutch slip" in our original boat when we first bought her, and could only get about 3.5 kts out of the (6hp) engine, but replacing the gear box improved that no end. We subsequently replaced the two bladed fixed prop for a slightly smaller two bladed folding prop, which transformed the boat's sailing performance (we started winning races, instead of coming last) and, interestingly, also helped under engine...

Sorry to say this sounds like a slipping gearbox!!!!-sadly most modern marine gearboxes work on the clutch principal-small hurths are mechanically locked but require little clutch discs to achieve this/larger gearboxes rely on hydraulic rams to keep clutch drums/dics in contact with one another and thus transmit power.
Oh and by the way they are reconditionable and if parts are required go to The US for them-they are cheaper despite having traveled over from Germany!!(in case of Hurth/ZF)
Phil