Optimal outboard mounting height and trim

[scugpg]

Hey guys, I'm looking to see if I have things setup correctly in terms of outboard height and trim at wot.

 

I raised my motor to the 3rd of 4 possible mounting holes and went out for a run. Top end was faster and rpms around 5800 (6000 max recommend) but noticed as I trimmed out a fair bit it started to ventilate. Still lots of water being pumped though.

 

Looking down at the cavitation plate at wot and trimmed out to fastest speed the plate almost looked dug in on an angle towards the front of the boat. So I'm wondering:

 

Do I need to raise the motor up one more hole and try it out - this way I can trim in more at wot to reduce that angle/drag and possibly some venting? (Presuming still getting water pressure and doesn't blow out in turns? Or does it sound like I'm close to the limit?

 

Just wondering what else I should look out for as I dont want to hurt anything during the tweaking process .

 

Thanks

[hirk]

Do you have a water pressure gauge? If not don't raise it and get one installed asap so you know what pressure you have now, seeing water doesnt mean anything.You may have been over trimming when you heard the "ventilating sound" , did the bow lift more when you hit the trim button after hearing it? It might just be increased prop slip from over trimming. What boat/engine/prop are you running?

[scugpg]

Do you have a water pressure gauge? If not don't raise it and get one installed asap so you know what pressure you have now, seeing water doesnt mean anything.You may have been over trimming when you heard the "ventilating sound" , did the bow lift more when you hit the trim button after hearing it? It might just be increased prop slip from over trimming. What boat/engine/prop are you running?

Thanks for the reply. Don't have a water pressure gauge right now. Didn't notice the bow raising when I heard the noise though so perhaps it was over trimmed. Its a 16ft tiller with a 60hp 4 stroke - 13p 3 blade getting around 5800rpm at wot. Hoping to get a bit more top end by raising if it can done safely. Could also consider a 14p.

 

Thanks

[bigbuck]

You are pretty close to the top wot rpms. You are looking at 1-2 mph at the risk of screwing things up. I would worry more about getting on plane with a full load. If you go up a number in pitch, you will drop a couple hundred rpm and possibly gain top end but you won't get on plane as efficiently.

[scugpg]

You are pretty close to the top wot rpms. You are looking at 1-2 mph at the risk of screwing things up. I would worry more about getting on plane with a full load. If you go up a number in pitch, you will drop a couple hundred rpm and possibly gain top end but you won't get on plane as efficiently.

I tested things out with full tank of gas and load today and got those #s. Motor has tons of hole shot and planes quick with the 13p. Perhaps I need to try a 14p.

 

Thanks

[DRIFTER_016]

You go to a 14P and you will drop to about 5600 rpms but should gain 1-2 mph on top end while losing a touch of hole shot.

[SlowPoke]

Ventilation is a result of not having enough blade in the water. I would consider similar pitch and diameter in a 4 blade prop. This will maintain your hole shot, perform better in turns and provide a touch more trim over a 3 blade prop. Peak performance is striking a balance between hp/weight distribution/water displacement and knowing when its as good as its gonna get.

 

You seem to have enough hp to support that pitch at that motor location.

Raising the motor will result in more ventilation and less ability to trim leaving more hull in the water (water displacement).

Lowering the motor will offer more trim but will require more hp to provide lift.

[scugpg]

Thanks Gents. I exchanged for a 14p and will try that out in the 3rd height setting or maybe bump it up to the 4th setting. Thinking the 14p will get me around 5600 with two people in boat and perhaps a touch more in the 4th setting.

 

4 blade sounds like another option but isn't a 3 blade better for top end?

 

In terms of monitoring water pressure, is a visual check and an eye on engine temps a sufficient way w/o a pressure gauge?

[bigbuck]

You are not talking about a 5-10mph difference at wot. It is only 1-3 mph. I would worry more about getting on plane efficiently as well as having a good cruise speed.

[SlowPoke]

Ya, the speed boat and bass boat guys use 4 and 5 blade props to slow down... LOL!!

Think it through...you're achieving optimal RPM and blowing out at high trim. Why would you want a higher pitch to reduce your RPM and raise your motor to further reduce you trim options?

A 4 blade prop in the water will outperform a 3 blade out of the water all day long. You want to promote bow and stern lift to reduce hull surface in the water. That is the purpose of a four blade prop.

The additional blade allows for more prop in the water per revolution at higher trim levels and is especially helpful reducing blowouts on hard turns.

I'm not a prop expert but a higher pitch 3 blade and raising your motor seems like a colossal waste of time and money.

[SlowPoke]

You are not talking about a 5-10mph difference at wot. It is only 1-3 mph. I would worry more about getting on plane efficiently as well as having a good cruise speed.

A bit of a generalization but not inaccurate. If you are reaching absolute peak WOT performance from a 3 blade and move to a 4 blade of similar diameter and pitch, you will scrub a bit of top end speed off in most applications but you will benefit from a little better hole-shot and possibly better handling depending on hull design.