Originally Posted by tranmkp
Fred and Rick
I like the way you think, with the 70hp (hauled it yeaterday - now clean bottom clean prop and clean rudder (flat plate) boat feels very slippery -
7.5knts @ 1500rpm. On the cam (top for power curve) 2700/2800rpm 11.7knts - with a wake so big you could surf on it...
on thought - from a drag perspective - wouldnt a foil shaped rudder be more efficient than a flat plate? Im not talking about turning efficiency?
2 blade prop? expalin why nto a 3 blade - 2 blade is easier to turn? Less drag?
Rudder - a flat plate is not a very good rudder. The best rudder has a thickness ratio of around 20%. If you reduce the length of the rudder, keep it at the same draft and make it thicker you can dramatically reduce the area and still achieve the same steering force. SO the thick rudder is ONLY BETTER if you reduce the area. The benefit will not be much - unlikely to measure any increase in top speed.
Prop - The prop blades become more efficient as their aspect ratio increases. AR is blade area divided by maximum chord. However you cannot just fit a a two bladed prop with narrow blades and get a better result. If the blades are not acting on enough area they will operate at a high angle of attack resulting in high prop slip. So if the prop is heavily loaded, it is better to have more blades. Most boat props are a compromise to reduce draft so blades are heavily loaded. At full throttle your prop is heavily loaded and efficiency has dropped off. On the other hand 11.7kts is a good speed for 70HP in that boat.
I would need to spend time to model the hull to determine the best prop but I doubt that you would have the room to swing it. At full throttle and speed of 11.7kts a 16" prop would have an efficiency under 60%.
Assuming the boat requires 10kW (13HP) to do 7.5kts you need to go up to a prop 3ft diameter and turn it at 400rpm to get above 80% efficiency. Even with this diameter a 4-bladed prop is better than 2-bladed.
There is not much point in trying to improve prop efficiency at the top end because the extra force generated is simply increasing the wake.
If you are after fuel economy then a smaller engine swinging the largest 4-bladed prop that you can fit at slow rpm will make a measurable difference to fuel consumption in the 7 to 8 knot range. Of course top speed will NOT be much more than 8kts.
The exercise to go through this is time consuming and I would need to know the current prop size, gearing and a good idea of hull shape just to confirm current conditions. Then it is a matter of looking at options like how big the prop could be.
Unless you are using the boat a lot it is probably more cost effective just to throttle back on the current set up and tolerate the inefficiencies. The hull is not particularly efficient anyhow even at 7kts.
These boats were designed and built in an era when hydrocarbons were cheap and use for liesure not regarded as anti-social. If you were designing now you would have a longer, more slender hull, canoe under water with a bigger prop and requiring much less power. How things used to be when engine power to weight was much less than now.