Attention propeller heads!

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james.brine
Well, as I said Rick, you are

Well, as I said Rick, you are welcome to borrow my ‘propulse’ adjustable pitch prop to eliminate pitch issues and then just buy the correct pitch for your boat. I’m pretty sure it’ll fit. It was bought for a tohatsu 50. I’ll have a look on their website later and see.

rick.jones
rick.jones's picture
Thanks James, very good of

Thanks James, very good of you. I'll get back to you this come the spring when I'm ready to put the boat back in the water. It would certainly be an interesting test!

I take it yours is a model 6902, that's the one for all Tohatsus in their list. They don't actually list the 25/30 HP 4-stroke range, but I think the prop fit is the same.

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

mike.clarke
A 9 inch pitch isn't a lot

A 9 inch pitch isn't a lot for a 9.7 " prop used in a high torque, low speed application like a Mac. If you are seeing high cavitation you need to increase the pitch to reduce the revs and improve the grip on the water. The Yamaha 50s fitted as standard to the 26Ms are a high torque motor that use a larger than standard prop with a larger pitch than would normally be fitted to a motor of this size. In the case of a Y50HT motor its a 13 5/8" prop with a 13 " pitch. If the propeller isn't slipping due to a failed cutlas bearing - The rubbery bit that links the shaft to the blade hub- and its deep enough to prevent air drawing down from the surface then it has to be cavitation due to tip over speed so you either increase the pitch to increase the torque or increase the prop diameter depending on availability. Motor revs falling are not a problem as long as its not too much. A stainless prop is expensive compared to an aluminium prop - about 3 to 5x for a Yamaha 50 and if you hit something with it it is likely to damage both the prop and the gearbox. I personally think its better to buy an aluminium prop and replace if its damaged.

I recently bought a Yamaha one for £80 on ebay compared to £180 from a Yamaha agency - both the old damaged one and the new £80 one have the same casting marks ! So suggest you shop around as it looks like a 12" pitch prop can be had for your motor for about £40 - 50 on ebay or other online stores.

A Macs boat speed has a lot more to do with sea state and the weight carried than just the motor and prop. We can manage 18 knots when its flat calm two up and with a load of cruising junk on board but this falls to about 12 knots just up on the plane when there's a slight chop running with the motor at full throttle.

Mike C - Tarka 26M

leigh.ross
leigh.ross's picture
Mike,

Mike,

i think you’ve got it backwards. Increasing the pitch will increase the slip, not reduce it. A lower pitch is better for a slower boat. A slower,heavier boat will require more mass accelerated less for efficiency. So a larger diameter , lower pitch prop is the way to go.

Just run the numbers. With rpm and time being constants then a lower pitch will move the boat more slowly. If that slower speed is the actual maximum speed of the boat, then max efficiency has been obtained. If the pitch is higher , then you’re just going to get cavitation ,not higher speed. The higher pitch prop just can’t transfer the power to the water

Its the same with drag racing. You are restricted in the power you can transfer to the ground by the characteristics and size of the tyre contact area.

Leigh Ross

Crieff

0777 558-4561

1990 MacGregor 26S Ptarmigan 

1992 MacGregor 26S Pelican 

mike.clarke
Hi Leigh,

Hi Leigh,

I agree with your comments if the prop is already over pitched but in this case it appears to be under pitched with the tip speed causing the cavitation. So if you increase the prop diameter then this will get even worse for the same rpm so the only option is to increase the pitch. As a (oversimplified!) rule of thumb for outboard props 1 " of diameter is equivalent to about 2-3" of pitch in terms of power required and will effect the rpm by 300 to 400 rpm. The problem is that the requirements for pitch and diameter are different for high speed and low speed operation. Most yachts like the Macs run at less then flat out motor speeds as against ribs and similar that tend to run at near max rpm more of the time. The reason for a large diameter prop and relatively large pitch is to improve torque - think about the huge props diameters run at low rpm on tugs. If the tip speed is high compared to the water moving past it i.e. low boat speed operation, then cavitation is going to be a major influence which is why I was suggesting a larger pitch to reduce the speed of the tip through the water as on most outboards you can't increase the diameter much because of the anticavitation plate clearance. Most outboards are supplied intended for relatively low torque, high speed applications which is why the Yamaha high torque motors fitted as standard to Macs were supplied with a lower gearing to swing a larger diameter higher pitch propeller.

An interesting discussion but of course the only way to really test the theory is to actually get hold of a different prop and see which way the change occurs. Theory is only useful if it works in practice.

Mike C - Tarka 26M

leigh.ross
leigh.ross's picture
Yup, my bad , cavitation

Yup, my bad , cavitation caused by tip speed could be the issue. I’m pretty sure a tech rep at one of the engine dealers has a chart that would so,be this conundrum.

Leigh Ross

Crieff

0777 558-4561

1990 MacGregor 26S Ptarmigan 

1992 MacGregor 26S Pelican 

john.pompei
Hi Mike,

Hi Mike,

I’ve been following this thread, and as you know I’m a new boy to Macs and as my outboard is a 50 Yamaha 2008 model its probably a similar spec. to yours.

Id be interested to know the maximum revs you can achieve flat our with the ballast in, the best I made was 4800rpm which gave a speed of 10-12 knots.

When I removed the ballast on a different occasion I made 15 knots with one crew on the way back from the Big Mac but didn’t clock the revs.

Another interesting bit of info was that I did some fuel usage tests before coming up to Bradwell and more miles could be travelled per litre at 4500 rpm at 10kn + that at 6knts . I guess this is because the boat is out of displacement at the higher speed.

Just some observations,

Regards,

John

ASHANTI2008M

mike.clarke
Hi John,

Hi John,

The published Yamaha F50 fuel figures for our motor are:

Flat out on a good day with calm water, little or no wind, two up we have seen 18 knots at max revs which is about 5400 rpm with the the high torque motor gearbox and the standard 13 5/8 inch x 13 pitch prop. But in most sea states we struggle to get above 14 -16 Knots when the tide is taken into consideration eg average of 2 back to back results in opsoite courses.

I tried to plot RPM against speed a couple of years ago but the day we did it was a bit choppy with a BF 3-4 wind so we couldn't get the full hull speed we get on a calm day but for the record what we actually found was:

We normally manage to cruise home on the Blackwater when the wind drops or if we are trying to make a tide at about 12 knots just up on the plane at circa 4000 rpm and 9/10 litres per hour consumption. We aso find we can motor sail home at a steady 6 knots with a much lower rpm and point much higher than under sail alone.

The later Yamaha 50s are fuel injected after about 2005-6 and more fuel efficient - ours has 4 carbs.

Hope this helps.

Mike C - Tarka 26M

john.pompei
Thanks for your reply Mike

Thanks for your reply Mike

unfortunately the figures you posted (? Spreadsheet) only appeared as a box.

My Yamaha is fuel injected high thrust but I don’t know the prop data. Next time Im at the yard I will attempt to measure it . The diameter is oK but how to tell the pitch - are there markings ?

I did a few trials and at 2900 rpm 6kn used 5l/hr and at 4000rpm 10 kn 8l/hr.

Regards

John

mike.clarke
Hi John,

Hi John,

In most cases on the Yamaha motors the propeller markings are unhelpfully cast into the propeller hub that faces the gearbox so can only be read by removing the propeller. A few have them on the base of the blade according to the manual but Ive never seen one that has when looking round the boatyard. I guess you can relatively easily directly measure the diameter though the pitch is more difficult to measure.

I've tried adding the two graphs I have as pdf files but not sure how it relates to teh fuel injected version of the motor. Your luck to have the fuel injected version. I had to spend a couple of days stripping down all four carbs and replacing the seals and float valves as when mine had a fuel leak this summer. A real pain to do hanging off the back of the boat to remove them all.

Interestingly the figures you give are very similar to the ones I have. Your 6kn at 2900 rpm is the same and I suspect our slightly lower speed at 4000 rpm is due to the being a bit of chop slowing us in our test. The fuel consumption figures in my graphs are from the US Yamaha site for our model of motor but the speeds are as we measured them in the Blackwater. The close similarity suggest you have the same 13 5/8" x 13" prop as both our motors are the high thrust versions which came fitted with the lower gear ratio gearbox and propeller with a larger diameter and pitch than the standard motor.

Thanks for hte data. I hope you can see the attached graphs this time.

Mike C - Tarka 26M

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