More tilt or less height

[ed72vette]

I have a 186 sailfish cc with a 90hp 2s Yamaha [2004]. The prop is a ss Power Tech 13.75 x 13[15YM90SCD3R13s] that gives that acts like 15 pitch prop. The motor is set in the next to the hightest hole so it is sitting higher than the manufacture recommends. He recommends one hole down. The boat runs fine, but if I trim the motor to the highest rpm my water pump sucks air and I loose water pressure. I can get 34 mph at 5200 rpm with the present trim before I loose pressure. If I tim the motor up further I can get 37mph at 5500 rpm. I do not get any cavitation in the present set up. Would I be better off lowering the motor one hole and being able to trim up futher or leave it the way it is now?
Ed

 

[JUSTINTIME]

Re: More tilt or less height

well if you sucking air drop one hole and check

 

[ed72vette]

Re: More tilt or less height

Does anyone else have a more comprehensive thought with some imagination as to what the trade offs might be; speed,fuel ecomony, ect?

 

[realmasa]

Re: More tilt or less height

Test it, usually the only way to find out.

 

[Esox]

Re: More tilt or less height

I would also lower it one hole. I would think that being able to lift a little more of the hull out of the water with the trim will increase efficency a bit more than having 3/4" less lower unit in the water. Loosing water pressure isn't a good thing. As was already stated - testing is usually the only way to know what will happen.

 

[ed72vette]

Re: More tilt or less height

Thanks guys for the response. I raised the motor up one notch this morning. however the rough water and wind will not alow me to test it today. I have a water pressure and temp gauge, so I was able to watch the pressure and temp when I was testing the tilt, so I am sure not damage was done to the motor. I have a fuel meter so I will check the milage,as well as the speed and rpm, in the lower hole compared to the upper one. I let you know what differences I notice.
Ed

 

[steelespike]

Re: More tilt or less height

It seems to me your not sucking air because of trim.The motor is too high.
As you stated it is all ready higher than reccomended.
I'm pretty sure there is a reason for that reccomendation.

 

[ed72vette]

Re: More tilt or less height

The manufactures sometimes give a hight setting that is a little consevitive to avoid problems with sharp turns and different props that can be tried. The only time I get some air if I trim more than half way in the up position. I am not experiencing cavitation in sharp turns or on hard excelleration. I am concern that my fuel consumption may go down. But as Esox said I might be better off with more trim and a little more drag of the lower motor. We will see.

 

[steelespike]

Re: More tilt or less height

I think that is right.You probably need to be able to trim more.
You could try a jack plate it would get the motor in cleaner water
and would allow fine tuning the height.
Not sure on the 04 but the earlier 90s the water pick up appears to be quite low.I wonder if your lack of blowout problems is a result of an efficient prop design not lack of height.I noticed on the illustration I have that the intake is a series of slots I wonder if blocking the top slot might solve the air sucking.As long as you have your gages you can tell if it hinders flow.
Its possible you would only have to block one or the other top slot.
Sounds a little off the wall but you seem to want that height

 

[ed72vette]

Re: More tilt or less height

I would like to keep the height even if only .75 inch., Barneget Bay is very shallow and you never know when you need the slightest of margin. I am nervous about blocking cooling slots. Grass is a problem now with the cooling slots if you are not watching your gauges. The motor is raised at this point, now all I have to do is test for results. Thanks for you comments.
Ed

 

[steelespike]

Re: More tilt or less height

If your running in tricky waters you may want reconsider the stainless prop.
Stainless doesn't give like aluminum and the gears/driveshaft may be the next weakest part.

 

[ed72vette]

Re: More tilt or less height

The bay is mostly sand and mud with clam shells. I had an aluminum prop when I first got the boat, the clam shell in the mud rounded all the edges. The ss prop in much more durable with only a small chance of hitting somthing substantial.
Ed