propeller problems

J

Joker

Well-Known Member
Joined
2 Jul 2010
Messages
1,079
Location
location location ...
Visit site
I've raised this in the past, so I apologise for raising it once again.

My boat is 30 foot long, and will reach 6.7 to 6.8 knots under sail without too much difficulty.

Under motor, it is a different story. At 2000 rpm, I would get about 4.3 knots. I struggle to get much over five knots, even at 3000 rpm (and I haven't really pushed the motor beyond that).

The engine is a Betamarine sail drive, 20 horsepower, with a folding two blade Radice propeller. The blades are 15 X 10.

I think the boat is under propped, and could do with a larger blade with a greater pitch – say, 16 by 12.

I would be grateful for any advice.
 
It would probably be helpful to push the engine to full revs just to see if it will reach the revs quoted by the manufacturer in the manual.

If the engine easily reaches full revs, as I expect it will, with the speed still under 6 knots then you definitely need to upsize the prop.

If you contact Darglow or whoever I guess they might ask for the speed at max revs.

Richard
 
I should have mentioned that the sail drive has a Selva fitting, which complicates matters. Most people who supply folding propellers only supply ones for a Volvo fitting.
 
Bigger isn't neccessarily faster. It has to balance the hull & engine. He might have suggested a smaller prop as suspects cavitation?
 
Whatever the reason, I would expect any half-decent 30-footer to cruise comfortably at about 6kn with 20hp. My Sadler 29 cruised at about 5.7 with 18hp, and my current 34 at 6.5 with 27.
 
One would have thought that a larger area propeller would move more water backwards – but it's probably not as simple as that, since you need to match the power of the propeller output to the power of the engine.

I think I will try to find some 16 X 12 blades and try them out. If they don't work, I can always put them onto eBay.
 
Joker said:
I've raised this in the past, so I apologise for raising it once again.

My boat is 30 foot long, and will reach 6.7 to 6.8 knots under sail without too much difficulty.

Under motor, it is a different story. At 2000 rpm, I would get about 4.3 knots. I struggle to get much over five knots, even at 3000 rpm (and I haven't really pushed the motor beyond that).

The engine is a Betamarine sail drive, 20 horsepower, with a folding two blade Radice propeller. The blades are 15 X 10.

I think the boat is under propped, and could do with a larger blade with a greater pitch – say, 16 by 12.

I would be grateful for any advice.
Click to expand...

Do we assume the hull is clean?
Modern (ish?) hull shape, not extreme overhangs or double ended? ( I assume fairly modern, due to saildrive!)
What displacement?
Can you clarify "struggle to get much over five knots," ?
If you are getting say 5.5knots at 3000 rpm and 4.3kt at 2000 rpm, is that not telling us that the drag has gone up a lot?
Before spending lots of money on new bits, I would look at the various prop calculators and see if you can make them fit what you are observing.
 
Looks a nice boat.
The best book I own on this subject is Larsson and Elliason's 'Principles of Yacht Design' . Or something like that.
It is a £40 book, but £40 does not go very far in a propeller shop.

If anyone has other favourite books let's share that.
I had an Impala 28 which did about 6 knots on a 10hp Yanmar, if you could stand the noise, but any chop would knock it back.
It was a boat to sail not motor!
 
I am returning to the issue of blade area. A propeller does not work like a nut and bolt. For every revolution the nut advances along the bolt by a distance equal to its pitch. If a propeller worked like that a 12 inch pitch propeller would push the boat a quarter of a nautical mile every minute at 3,000 engine revs with a 2:1 gearbox. Thats 15 knots! What a propeller does is generate thrust. The boat needs a certain amount of thrust measured in KGf at any given speed, and the ability of a propeller to generate thrust is dependant on three things, its blade area, its diameter, and its pitch. Imagine a three blade prop with large wide blades, and one with narrow ones; which will give the most thrust? or if the boat is tied to the shore, the most bollard pull?

The answer to the above is obvious, and that is why I think that the limited blade area of a two blade propeller is one reason why the OP's boat fails to achieve the expected speed. A simple swap to a three blade propeller may be all that is needed.
 
Norman_E said:
I am returning to the issue of blade area....
The answer to the above is obvious, and that is why I think that the limited blade area of a two blade propeller is one reason why the OP's boat fails to achieve the expected speed. A simple swap to a three blade propeller may be all that is needed.
Click to expand...

I've often wondered why blade area never features as a parameter in any of the prop calc algorithms. Even for two blade props the difference in maximum revs is noticeable between a 'skinny' two blade and a 'fat' two blade.

The answer may be obvious and the swap simple but the move from a 2 blade folding prop to a three blade folder is a hell of a lot of money.
 
Unfortunately, the sail drive has a Selva fitting, which is very rare these days. About the only people who make a suitable propeller boss are Radice. I have tried various other suppliers, but they all come back saying no. I am stuck with the propeller boss I have at the moment, and the only thing I can try is to change the blades. Or buy a new sail drive!
 
You must log in or register to reply here.

Share this page

Top