Attention propeller heads!

23 posts / 0 new
Last post
rick.jones
rick.jones's picture
Attention propeller heads!

This is actually a serious query, but I hope I've got your attention! The problem is my outboard propeller is behaving in ways I don't understand, and not giving the results I should get from the engine.

The basic problem is that the propeller is not "gripping" the water, or in other terms I'm getting massive prop slip - 50% or more.

The engine is a Tohatsu 30HP, optimum rev range at WOT is 5250 - 6250, gear ratio is 2.17, and is mounted on a Mac 19. I've fitted a 9" pitch prop, which should be good for 15-18 kts without ballast. At 18 the slip would be 10%. In practice the max is 11 kts, with the engine spinning at 5750. That's the ideal revs, but the prop slip is around 45%. That makes no sense to me.

Something else that seems odd is that if you open up the engine to WOT with the boat stationary, the revs hit 5750 immediately, before the boat's even started to move. It's as if the prop's just not getting any traction in the water.

Some things I've read suggest that I need a prop with a bigger blade area, either larger blades, and/or a 4-blade. I wouldn't have thought increasing the pitch would do much, it would either overload the engine or lead to even more slip.

I think the prop height is OK, I've attached a photo of the motor fully down, taken at the level of the anti-cavitation plate. It's definitely well into the water. The plate is below the bottom of the hull - can a prop be too deep?

I'd appreciate any thoughts from the technically minded, and those with experience of messing with props. I'm tempted to go for a stainless 4-blade, but it would be expensive, and a lot of wasted money if it's not the answer!

Thanks in advance.

Rick Jones (Treasurer) 1994 Mac 19 "White Lightning", Isle of Wight

dave.newton
 

Random thoughts:

The position looks good, well clear of the hull. I've never heard of any argument that a prop can be too deep other than problems of excessive leverage from a long shaft engine on an insubstantial transom. Not really an issue for a Mac as the transom structure is rock solid.

Is it a problem with trim? Is the prop working hard to lift or sink the stern rather than push it forward? That would explain the high slip.

It looks like a rather small prop for 30HP, I'd have expected to see greater surface area, either from bigger diameter or more blades. That's simply based on the typical sizes I've seen on various RHIBs etc. But they have a low mass to wetted area ratio compared to a Mac.

Are there any trim / prop walk plates missing? That might screw up the intended flow around the prop.

Is the prop on backwards? Doesn't look like it and not normally possible to do, I'm clutching at straws now!

Dave.

Dave Newton Sailbadthesinner

leigh.ross
leigh.ross's picture
What’s the diameter ? A very

What’s the diameter ? A very short search for outboard props for 30hp outboards shows a range between about 9-12 inches. If the engines revs up immediately without moving the boat I’d suggest that implies too small a prop.

Leigh Ross

Crieff

0777 558-4561

1990 MacGregor 26S Ptarmigan 

1992 MacGregor 26S Pelican 

rick.jones
rick.jones's picture
Hi Leigh

Hi Leigh

Diameter is 9.7", Tohatsu stock. You're confirming my own suspicions, I need to look for a larger diameter and/or blade area, i need to measure the clearances to see how big a one will fit!

Rick Jones (Treasurer) 1994 Mac 19 "White Lightning", Isle of Wight

rick.jones
rick.jones's picture
Hi Dave

Hi Dave

Thanks for your thoughts.

I don't think it's a trim problem, the prop shaft is pretty much dead horizontal. There's no plates missing, and I'm sure the prop's the right way round - as you say it's pretty much impossible to fit in reverse, this is how it arrived from Tohatsu (it was new last winter).

It's Tohatsu's stock 9" pitch prop (9.7" diameter), and I agree that it seems quite small, in diameter and area, which is my thought as to what's wrong. There's a good clearance below the plate to go at least an inch bigger in diameter I would have thought. I forgot to measure that, must check when I next visit the yard.

Is there any minimum recommended clearance between the edge of the prop and the plate?

Would you recommend anywhere particular as a supplier of props?

Thanks.

Rick Jones (Treasurer) 1994 Mac 19 "White Lightning", Isle of Wight

james.brine
Just a thought rick bit is

Just a thought rick bit is the prop not the problem? Does the engine have some sort of slipper clutch in addition to the shear pin to prevent damage in a grounding or entanglement situation. Is it positively going into gear? Is the prop turning on the shaft somehow? I can’t imagine it can move on the splines but I have known the rubber ‘cush drive’ centre of the prop come apart from the body of the prop. Try putting it in gear and trying to turn the prop by hand. Might give you an indication.

rick.jones
rick.jones's picture
Hi James

Hi James

I'm pretty sure that's not the issue, I'll double check next time I'm at the boat though. There's loads of water churning when the prop's spinning, so I don't think it's slipping on the shaft. I've done over 50 hours on it already, so if that were a problem I think the hub would have disintegrated by now!

Rick Jones (Treasurer) 1994 Mac 19 "White Lightning", Isle of Wight

david.sandwell
Rick

Rick

My initial thought was that perhaps the propeller was slipping on the rubber insert as James has said above. The cavitation plate should be 0.2 to 1 inch below the bottom of the boat any deeper produces additional drag. The angle of the motor to the transome should toe in about 12 degrees to get the boat nose down and lift it up onto the flatter stern area. I looked up the performance figures for a mac 19 on the macgregor site and it said 25 mph with a 40 hp motor another site said 25 mph with 50 hp motor so we are looking at 22 knots. The waterline length is 17 feet so max displacement speed 5.53 knots, semi displacement up to 10.3 knots then boat begins to plane, at about 10 knots it needs all the available power to get the boat out of the hole. I tried to find any posts on the web site for mac 19 and potential speed the only mention I could find was 11 to 12 knots maximum with a 25 HP evenrude etec. This would be a 2 stroke that delivers lots of torque, it is normal practice to go an engine size higher in HP when going from a 2 stroke to a 4 stroke engine to get equivalent performance.

When I took my aluminium propellers to a prop shop to get them repaired I asked what efficiency I should expect. The reply was about 45% in displacement mode and a maximum of about 70% when planing on my X the prop efficiency is about 65%. Looking at American mac site and actual performance figures mac M with 70hp suzuki 64% propeller efficiency, Mac M with 100hp 64% at lower speed and 82% at 23 knots . For your boat the prop efficiency is about 56% which is quite good for a power boat just abut to plane where maximum turbulence is being created around the hull.

When doing a safety boat coarse we took out a 4.6m rib with a 30 hp yamaha motor, It would not plane with 2 big boys and the instructor on board, the bow came up and it ploughed through the water. The next day we took out a similar rib with a 40 hp yamaha this time the boat lifted out of semi displacement mode the bow came down and it reached 22 knots shown by my GPS. The first boat just did not have the power to get out of the hole.

A 9 inch pitch prop rotating at 2650 rpm would achieve 19.6 knots at 100% efficiency or 12.8knots at 65 % efficiency. The answer to higher speed is increase available HP and back off power when on plane.

rick.jones
rick.jones's picture
Hi David

Hi David

Thanks for your detailed thoughts. Fitting a modern motor on a 19 is actually a bit of a challenge. When the boat was designed in the mid '90s, a 40HP was a compact 2-stroke machine. When I bought the boat it had such an engine, but it was pull-start and manual tilt, and a typical 2-stroke pain. I decided to put a modern engine on it, but all current 40HP engines are de-tuned 50s, and hence bigger and heavier.

I actually bought a Tohatsu TLDI 50, the same that I'd used previously on my X and been very happy with. It's the lightest 50HP available (but 95kg even so), and the only one where the power head is small enough to fit the 19's engine well and still turn. It provided the power, but was really physically too big - it wouldn't fully tilt up, and was rather too much weight on the stern. It was also very noisy - in the 19's cockpit you are very close to the engine, much more so than an X or M.

I began to think last winter that it might be better to sacrifice power for a quieter ride, less stern weight, and a more manageable motor. As it turned out I got a good offer for the 50, and my boat yard did a good deal on a new 30, so I was able to swap engines for quite a small outlay. Overall I'm pleased with the result, but I just feel the motor should be able to transfer power to the water more efficiently. The fact that the engine spins at full revs regardless of the boat speed is what makes me think the prop is not doing its job.

Trim is also a bit of a problem on the 19, as its transom is pretty much vertical. The motor is mounted on wedges, but it still doesn't trim down as much as I'd like. I'm sure it would help if it would. Maybe I need more wedges!

I can't help thinking a 4-blade stainless prop would help, Solas do one, but it's about £200! A lot of money if it doesn't make much difference.

I will keep chewing things over ...

Rick Jones (Treasurer) 1994 Mac 19 "White Lightning", Isle of Wight

simon.armitage
I am no expert on outboards

I am no expert on outboards or props but, Rick, was it better earlier this year? Was the change sudden? Is it possible to think back through the year's sailing to try and identify point when the output dropped and then investigate any changes. You have the winter to think about things.

Not much technical help, I'm afraid.

Simon

rick.jones
rick.jones's picture
Hi Simon

Hi Simon

No, nothing's changed, it's been like this since I fitted it. I've just been chewing it over more now it's winter and there's more time for theory than practice!

Rick Jones (Treasurer) 1994 Mac 19 "White Lightning", Isle of Wight

Pages