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.

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

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

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