spun prop hub or ventilation??

[bkd13]

I have a 18 foot 77 allison craft with a 70s model 1150 merc 115hp with 2 blade 21p prop I just got the new motor put it on and when I took it out for the first time it was doing great I got up onplane and when I went to try wot the motor tilted forward and overreved and started loosing speed but would go back to normal when I let out of throuttle a little. No power trim but it was set on lowest trim setting. Any ideas?? Thanks

 

[smokeonthewater]

Re: spun prop hub or ventilation??

could be either.... make a mark on the end of the prop shaft and on the prop.... then run it till you get the prop again..... check to see if your marks are still lined up

 

[Texasmark]

Re: spun prop hub or ventilation??

I would think that your maximum load on your prop would be coming out of the hole....getting on plane, especially when you first put the pedal to the metal. Do as smoke said and also check where your antivent plate is with respect to the water coming out from under the transom......can you see the plate when running as fast as you can? If so, how much of it..1" or more above the level of the water flowing past?

Look at the hull from the stern. Is it straight or does it have a hook in it from sitting on a poorly matching trailer. Are there any hull obstructions in front of the prop that could cause water disturbances that would result in ventilation.

The thing that bothers me and makes me think it is the prop is your comment about being on the lowest tilt setting....meaning that the bottom of the engine (lower unit) is as close to the transom as you can get it. If you had it tilted out as far as it would go then ventilation could surely be a question.

Mark

 

[bkd13]

Re: spun prop hub or ventilation??

Would a 3 blade prop keep a. Better bite on the water if it is ventilation over the 2 blade?

 

[Texasmark]

Re: spun prop hub or ventilation??

Would a 3 blade prop keep a. Better bite on the water if it is ventilation over the 2 blade?

Probably and if it were SS high performance with cupped blades it surely would. Of the two things I mentioned cupping has the most impact on grabbing the water and holding onto it. The rest of the HP equation is high rake which lifts the bow decreasing the wetted area of the hull thus reducing drag and increasing speed and the SS has thinner blades which cut the water more easily resulting in less drag and higher speed....then some go one step farther and port the prop (drill holes in the barrel near the front of the blade) which allows ventilation on the hole shot which improves the hole with a higher pitched prop that would give you higher top speeds than you could get if you had to prop for the hole shot alone...sell them on site.

Mark

 

[ENSIGN]

Re: spun prop hub or ventilation??

You said the motor tilts up.It sounds like the locking mechinism is misadjusted

 

[bkd13]

Re: spun prop hub or ventilation??

The motor does tilt up when it starts over reving but the motor isn't supposed to be locked down when in forward incase you hit something correct??

 

[smokeonthewater]

Re: spun prop hub or ventilation??

correct the engine springing up is not part of the puzzle.... It's just losing prop thrust and springing up a bit... kind of like letting go of a rubber band.