Running 4K at WOT

[aj10203]

Hello everyone. I am looking for prop advice. I have a 1989 Larson San Marino 250. Mercruiser 260 w/ alpha one. It is rated at ~5k lbs from the factory. I am guessing with a full fuel tank (80 gals), all gear and passengers (usually 3-5 besides me), I am around 7000-7500 lbs. Does this seem reasonable/correct? It currently has a Mercury High five stainless steel prop. PN# 48-815758-19. 13.25 diameter by 19 pitch 5 blade. I am happy with the performance of this prop, EXCEPT that I can only get to 4000-41000 RPM at WOT. If I buy the same exact prop in 17 pitch will I have similar all around performance but with the increase in rpm to where I need to be? Would a 4 blade possibly be better? The boat came with some 3 blade props, but it took forever to get on plane.

 

[Scott Danforth]

you have too much prop currently or not enough motor for that prop with that hull. at 4100 RPM, you are lugging the motor.

were any of your 3-blade props vented? venting will get you out of the hole faster. i would expect a 3 blade, 15 x 17p as a good starting point

the mercury prop selector is recommending the Black Max, Revolution 4 or Vengence

 

[jimmbo]

Being that the boat and motor are 30 yrs old, I would be checking to see if it is capable of making its power. A Compression Test is in order, verification of Spark to all cylinders. And, that engine has a 4bbl carb, probably a Quadrajet, but possibly a Carter/Weber, and some assurance that the Secondaires are opening enough to maximize power, and there is also enough fuel passing through.

If everything is in Order and up to snuff, then you do have a prop with too much pitch. In 89 the WOT range for that engine was 4200 - 4600 rpm

A 25 footer, is not going to be a ball of fire when it comes to holeshot with a 5.7. What is the gear ratio of the Outdrive? You should be able to find it on the Trim/Decal surrounding the Upper Gear Case

 

[aj10203]

I do not believe any of the 3 blade props I have are vented. They are as follows, all aluminum:

Mercury Black Max 13.75 x 21
Mercury Black Max 15.25 x 15
Michigan 031024 14.5 x 17

I will try the 14.5 x 17 and record performance numbers.

 

[aj10203]

Jimmbo, I believe the engine is up to snuff, but I will check everything you suggested to be sure. It has a Quadrajet I rebuilt last year. The outdrive gear ratio is 1.5:1. I am fully aware I wont be winning any acceleration records lol. Nobody tubes behind me nor do I pull any skiers (I doubt even propped for holeshot I could haha). I just want to maintain my current holeshot performance while getting the engine into the recommended RPM range.

 

[QBhoy]

You have far too much prop as a 19”. Other than that. The high 5 is not a prop for your boat.
Your ratio would most likely be a 1.47/1.5 probably.
Most boats I know similar like larger bayliners 26-28ft with same engine and drive would come with a 15-17” prop standard.
Most of these guys here have changed to a 16x16 3 blade with great results. Loose a little too end occasionally but transforms the boat all round.
To put it into perspective. I know of 20ft bow riders with this engine and drive. They turn a 19” at around 4700 rpm at a push.