Re: swapped prop for same - altered height - revs through the roof
If wanting to achieve the best power out of that 140 Tohatsu, with well ballanced boat, and cav plate riding paralell to water surface on plane, water flow should pass slightly under small upper water deflector plate. If it's a long shaft water flow is probably hitting the round non edge upper portion of middle leg, you could have unwanted water splashes at back transom which will slow you down a bit.
The ideal tail height must be visually checked by a transom spotter, then dial best transom/engine height for that engine, play with height positions untill opt leg height is reached, then you can play with all prop pitches you like.
Happy Boating
Thanks Sea rider and thanks for the PM, i'll try and digest the info. Hopefully this weekend i'll be able to do some mid loaded testing on flat ish water with a transom spotter. Prop was purely bought due to the old one being destroyed and in theory should have been the 'same'. how silly I was to think that!
well I understood your prop was destroyed but the idea I was saying was to buy the listed exact replacement tohatsu prop for your motor and not use the Yamaha which isn't the correct brand AFAIK and may give you a false impression of the results. I think you may have had the correct prop all along (or close to it) but you just had the motor set too low for best performance.
I am of the understanding that different brand props use different rake and cupping designs to match the power and gear ratios they use so performance can vary in a dramatic way between them.
as to cupping, yes cupping is a very big deal and is as important as pitch and diameter.
as to motor height, my intention was to guide you to return the motor to the factory position as best as you can (cav plate even with or just above the bottom of the hull) and then and only then decide based on the way the correct prop (the new tohatsu prop I suggest you buy) works, and then decide based on how it performs, if you have the right prop or not.
if you continue to get blow outs and the test results suggest you do need to lower the motor position, you should only move one hole at a time and never two because that is way too much of a difference.
Thanks Limitout. Same prop isnt an option due to the difficulty trying to get parts at a reasonable cost. the whole idea is trying to set a control / benchmark to then work out where to go with a final prop. I'm going to get the 2 props measured and see what the difference is. The same company can add cup if required, I know its not the best way of going about things, but its something that gets us boating again (we do about 200+hrs a year runnning).
The motor is now at its factory height so i'll work from there. Ventilation plate is slightly below level with the bottom of the hull (not the small keel which sits about 3/4" lower) One indicator that it was too low was the forward spray I got from the leg at speed. I could reduce it by trimming up off the gauge and this was the point i got max revs.
You may be able to retain the height with a different prop but it seems hardly worthwhile trying to get to the cutting edge when it's a fishing boat going over 2M swells & chop.
Take a look at some quintrex boats they cut the top off the T and in addition they tapered the T down towards the transom.
I remember reading on the Mercury website to expect a 1-2 mph gain for every inch raised, measured at 80 mph. I've also seen graphs where below 40 mph it makes sfa difference.
If it's low enough to avoid the keel bubbles at WOT then you won't have any ventilation problems either.
Thanks Fed, not trying to get to the cutting edge, just doing what I normally do, trying to get things just right. Its true that most of the time the boat will rarely exceed 25 knots on the water and in bad conditions no more than 15 knots. But we do occasionally have flat out runs when the weather allows. I'm still waiting on the trailer chassis back from the Galvanisers so once its back i'll photograph the keel profile at the rear. It should be fairly similar to the quinnie as the come out of the same telwater factory.
From what you've said you never has a "ventilation" issue but rather a "turning tight while trimmed for max speed" issue which you mentioned you suspected to be the case....
there is no such thing as a manufacturer making a prop that works best for their torque curve and gear ratio.... it's all about power, rpm drag, speed, and handling characteristics of the particular boat motor combo..... true props need to be correct for the motor and boat combination but having the same brand motor and prop is NOT important..... maximize the setup with your current prop and THEN look into what changes are likely to have the best results in your next prop
Thanks Smokeonthewater. Thanks for your PM too, I tried to reply but your inbox was full.
You are correct no real ventilation issue originally just ventilation on turns, which i now attribute to my newbieness to boating at the time. 6 years on I have learnt a lot.
rather than maximising my setup I think the best I can hope for is benchmarking it and then determining where to go from there. At least we can use the boat currently. Once testing is done i'll order a new prop. If that prop isnt right, i'll sell it and get a new one and so on until its right. Its a long winded process tho.
The prop measurement may also give some indication, I cannot find much about the cupping on tohatsu or yamaha props, but the tohatsu props were made by another company and apparently did have it, and I think the ally yamaha ones didnt.
I'll let you know more when testing is done (unless i get pictures before hand) I'll also see if I can post some prop pics for the expert eye to spot any differences.
Thanks again for your continued help!
Steve
