On this subject of props. the thread has swung from under prop to over prop as thread creep tends to do. This is fine in this instant as I’d like to submit a over prop presentation.
To day I took our 18 foot Poulsbo out for a halibut fishing effort. As I was leaving the harbor the subject of over prop came to mind and I took the opportunity to go WOT on our 6 hp Yanmar one cylinder diesel Hrumph! Max RPM was 1100. (Rated for 3000 RPM) Has been this case as long as we have had the boat/engine some 30 years. Never gave it a thought as the boat runs fine in all aspects. Yes, at anything over 900 RPM the exaust becomes very black!
We operate this engine at 700 RPM which would equal the 3/4 of available RPMs were the 1100 RPM to be that operating range not the 3000 RPM Yanmar rates as WOT
Okay. Now we know we are over propped. Upon return and the boat out of water an inspection showed the wheel is 11 inch in diameter and 13 in pitch. So off to Vicprop formula with this information. Here we go:
WLL=16 feet, beam 4’.6” draft, 1 foot. Wt: 1500# engine-6 hp Yanmar diesel,RPM rated; 3000 RPM 2:1 reduction plus the remaining data required. End result by Vicprop is wheel 11’ diameter X 8.6” Pitch. for a hull speed of 5.3 knots As a result we are 4.4 inches over propped for the boat. Extrapolate that difference out to our larger boats and it is a huge amount which would fit the broader discussion on this thread as to the potential outcome of over propping.
What is the running situation with this over prop? At 700 RPM the hull speed by GPS is 5 + not over 5.4 knots. Our GPH fuel burn is 20 NMPG per gallon. Engine temp must be within range as the indicator is a red light if over temp. Not happening. Salt water cooled which may affect that measurement, exhaust is cool to the touch and good volume. As to idle, the engine idles at 400 RPM and yes, we are moving at salmon trolling speed no doubt. About 2 knots per hour, however manuvering is quick and acturate making a very responsive boat during and leaving the moorage!
Annual usage of this boat is light with no more than 25 hour per year so the subjective damage of over prop has, will, take more time than normally allocated to that see the effects of wearing.
One has to consider that the engine is 30 plus years of age in a 78 year old boat.
I suppose the outcome of this post is that the little Yanmar over propped as it is, seems happy to be under the extra stress the over prop is pressing the engine . I can add to this that when I hook this boat to a fire wood log off the beach, of some length say 30 foot by 18 inches in diameter, the towing speed is closer to 4 knots vs. the 5 + knots running free, no change in engine operation, which represents the plus factor of having a over prop. issue.
Would I consider changing to a more compatible wheel measurement to Vicprop formula? I think not.
Cheers,
Al-Ketchikan (Bridge to Nowhere) Alaska