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Old 22 June 2014, 13:32   #71
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This is the water trial result on the Yam 25 L at wot under flat calm water conditions. Water flow passes slightly above AV plate, hydrofoil barely is doing it's homework as needs more water height for prop to grip better and plane faster that rib. Engine loses best prop thrust and ventilates badly on choppy waters and close turns at wot and speed.

On straight courses works aceptable, that rib is an old heavy model with a strange hull design, is transom rated for a 40 HP engine. Being way under powered with just a 25 HP, engine needs its full top prop thrust to work more efficiently.

In order for that rib/engine to work as owner expects to do so, transom must be chopped down at least 4 cm and test again to check if at Sweet Transom Height Spot. Owner understands the tech issue involved and clearly agree to chop transom down.

Happy Boating
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Old 23 June 2014, 12:35   #72
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Chopping transom is major surgery. Start with removing hydrofoil. Why have this on console boat. Go for sea trial. What's the RPM, speed, and prop pitch. Why not start with something less irreversible such as finding correct prop. What's the boat weight?
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Old 23 June 2014, 21:35   #73
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Sibrider,

Gee, with all respect forum members deserves, what have you been smoking lately, is someone advising you or it's your own tech thoughts on the matter. What you have said has no theory or practice ryhme or reason at all.

As you are expecting an answer, must confirm that planes excatly at same lower unit level stated height with or without hydrofoil, hydros are useless. Will be removed to fan and freshen my staff menbers, it's still heated down here, el Niņo issues.

We happen to count with a mayor heart surgeon, he can raise or chop down transoms on anything that floats, except bathtubs.

BTW, assay is ready, check once complete posting is over. I'm sure will enlighten your inflatable/engine setup and boating skills.

Happy Boating
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Old 24 June 2014, 00:12   #74
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I gotta say that the idea of trying the boat with a different prop seems much less dramatic than chopping the transom.

Furthermore, the boat only ventilates in choppy seas or hard turns, so maybe a different prop would help under these limited conditions while it performs well otherwise. From what I've read, more cup, more rake, stainless steel, and 4 blades rather than 3 all may help to reduce ventilation.

From what I've read, a different pitch has not been mentioned to reduce ventilation.
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Old 24 June 2014, 15:10   #75
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Simple answer : would you rather invest $ 150-200 on a new prop which will not work no matter what you've read or go for a 1.5 inch transom chop down that only costs no more than $ 80.00 and put you straight at the sweet tansom spot in 4 hours max.

That's the average time it will take to remove engine, remote cables, chop transom down, level and replenish with fiberglass what's been cut down, let dry, install back engine and steering, remote cables.

With the money you'll be left over by not buying a new prop, you can buy lots of beer cases and go boating all day long. We raise and chop transoms down on permanent basis.

Before you play with props, pitches you must know the wot rpm numbers achieved with that combo, and that's with an induction tach and lightly loaded rib.

Happy Boating
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Old 24 June 2014, 15:48   #76
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Quote:
Originally Posted by Locozodiac View Post
Simple answer : would you rather invest $ 150-200 on a new prop which will not work no matter what you've read or go for a 1.5 inch transom chop down that only costs no more than $ 80.00 and put you straight at the sweet tansom spot in 4 hours max.

That's the average time it will take to remove engine, remote cables, chop transom down, level and replenish with fiberglass what's been cut down, let dry, install back engine and steering, remote cables.

With the money you'll be left over by not buying a new prop, you can buy lots of beer cases and go boating all day long. We raise and chop transoms down on permanent basis.

Before you play with props, pitches you must know the wot rpm numbers achieved with that combo, and that's with an induction tach and lightly loaded rib.

Happy Boating
What makes you so certain that playing with pitch would not help?
I was suspecting budget reasons for reluctance with prop matching: expensive parts and cheap labor. I would suggest trying that none the less.

USA is very much reversed. Props are cheap at $60-70 for small alum. 4 hours in shop will set one back $300-400 easily.
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Old 10 July 2014, 12:23   #77
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Yep, cheers, will probably leave it as it is with height as per pic and standard prop - it picks up really well and 18 kts is fine really.

Not had my tiny tach on it yet to check WOT RPM but will soon.
Finally got round to doing some runs with the tacho on.

Mariner 15 with standard factory prop.

Best trim position was 2 and achieved 5750 rpm max at WOT. Touched 19 kts lightly loaded with myself 12 stone and young son around 5 stone. 19 kts was brief with behind us, average WOT more like 18 ish.
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Old 10 July 2014, 15:51   #78
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Sounds great. Actually fact that you are able to reach practically max WOT RPM (5200-5800?) tells you got some spare capacity in you motor/boat combo. Especially if that's your typical load. I believe mariner 15 horsepower is rated at 5500 RMP. Horsepower starts dropping when engine goes past that. You can up a pitch by 1 inch or so to go faster if you want to then.
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Old 10 July 2014, 16:27   #79
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I thought max on that engine is 5500 to 6000 so spot on at 5750 and no more to give. AFAIK??
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Old 10 July 2014, 23:18   #80
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Guess we need to be more specific. I looked up this one http://www.mercury-marine.eu/cache/D...cumentFull.pdf
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