Prop size and speed

My manager is big into boating (not fishing , posey speed boat stuff). He was saying that if I changed the size of the prop it can affect the speed of the boat. He reckons if you increase the speed by changing the prop size, that you lose a bit of lift (which I can live with as it's not lifty anyway) - anybody know anything about this ? He was saying about fuel consumption being better also. Anyone with any info on this, can you let me know, I'd change the prop to gain a bit of speed and/or improve fuel economy, but am not sure whether the prop size goes up, or down to do this. I can't ask the boss now, as I told him to stick his job, and now work elsewhere (no more watching TV for a living !! ).

Well, my knowledge of boats is limited to the plastic ones you play with in the bath. But what you say sounds like getting something for nothing. Surely you have to pay for extra speed in some way?

Teacher, I don't mind not getting any more speed out of the boat as I'm not racing it, but I'd be very interested in fuel economy.

You never get it for nothing as you have to buy a new prop and that can set you back about €150 yo yo's

I have heard of this before Conor. My guess would be to bounce this off a prop / engine supplier. I'm thinking marine parts direct.ie. They should know their business. They will be looking for boat size and weight plus engine size and type.

Kev

As far as i know its all about the angle of the blades on the prop, i dont know how much more top speed it will give you but a more aggressive pitch will increase acceleration and a less aggressive one will be slower. I think that the lesser pitch will give better fuel economy as it would take less power to turn the prop through the water, less resistance. Dont think the actual size makes much of a difference, a smaller one would be less efficient wont displace enough water to move the boat and a bigger one mightnt fit you engine. There are adjustable props that can be moved to improve acceleration and other things. http://www.propulse.se/.

There you go hope it helps.

Jack.

It's all a trade-off between speed, accel etc. There's plenty of sites that give info. like: http://www.boatfix.com/how/props.html

I don't think you'll make much of a difference in economy by changing the prop. Try not pushing your right hand down too hard!!

Small boats are expensive to run. We class ours as doing just under 1 nautical mile per litre of juice (this works out at just over 5 statute miles per gallon). Cruising will help but its hard not to go flat out if conditions allow!!

Jim

Thanks Lads,
After all that, I reckon I'll leave it, not broken , don't fix and all that craic !

a prop works like a screw going into wood.

they have a pitch which (notionally at least) is the distance that the prop will push you forward with each turn.

if you increade the pitch of the prop each revolution will push you further forward.

so far so good! but as pointed out you get nothing for nothing.

a steeper pitch needs more power, (if you think of it as a gear try starting your car in top gear - yes its faster once you get it to maximum revs but will you ever get it tomaximum revs?)

the optimum pitch for your prop is the one that will allow your engine to rev to max revs but NO MORE(plus or minus an inch or two for tinkering).

those adjustable props are not a great sucess by all accounts except on smallish engines due to a lack of stiffness of the blades

Conor if you are that concerned try giving seahog a ring and ask them. Boats have a particular criteria that affects that boat. It is not the same for all boats. For example. Most boats have engines with a 13" pitch between the blades. This would be the standard. Most engines would come with this. A 13 will give good acceleration and speed. A 14 will give better top end speed but be slower on the acceleration. The Raider uses a 17 " prop. This is the standard for this boat that works best as advised by Raider themselves. I would get over 2 miles to the litre with a 80 hp 4/stroke. That is travelling at a speed of 24 MPH. That is pretty eceptional. I have recently added a dole fin but have yet to use the boat with this on to see what difference it makes (if any). Try experimenting with your planing speed and the angle you set your engine. This makes a massive difference on fuel economy. If i hit full throttle fuel economy suffers but is still good. If you get the engine size matched to the boat that also is a huge factor. As i said ring Seahog (I think you have a Seahog anyway) and see what they say. They might even have performance figures.

My dad had dole fins on his 100hp yamaha it helped the boat get on the plane a lot quicker, the engine was really too big for the boat but it helped stabalise it when accelerating hard. the boat was a zodiac rated for a 90hp.

cheers again folks, might just ring Seahog and see what they recommend.