Suzuki 20hp … add a bit of power

[guinnessMD]

Hi guys.
Is there any way to add 5-10 hp to my Suzuki DF20AS?… have bought it for my inflatable 4.5m but it is not enough for it … manufacturer spesified max power 35hp, and I thought that 20hp will do… have changed the prop from 9 1/3 12 to 9 1/3 10 and got a bit better result but can go just half of throttle because after that engine goes to higher revs pushing the water back and not the boat forward (boat losing speed).

 

[99yam40]

Sounds like the motor prop maybe sucking air or some other thing is causing adisturbed water at the prop.
maybe motor is mounted to high, need to get the prop farther into the water

 

[airshot]

Once you get the prop "bite" figured out, you need a tach to find out max rpm at wot....otherwise your just guessing..

 

[guinnessMD]

Sounds like the motor prop maybe sucking air or some other thing is causing adisturbed water at the prop.
maybe motor is mounted to high, need to get the prop farther into the water

Hi… I thought that as well but the motor is way down in the water proper wise … the mounting board is 381cm in height and suzuki require that height as well.

 

[guinnessMD]

Once you get the prop "bite" figured out, you need a tach to find out max rpm at wot....otherwise your just guessing..

At the middle of throttle i have around 4400rpm… after that the shite happens.

 

[99yam40]

Hi… I thought that as well but the motor is way down in the water proper wise … the mounting board is 381cm in height and suzuki require that height as well.

where is the cavitation plate in reference to the bottom of the boat?
Maybe that motor is a short shaft and you need long shaft.

 

[airshot]

Unless I screwed up my math, 381 cm is around 150 inches tall.....that would be one really long shaft !!
Now, if you meant 381 mm then yoy woukd need a 15" length or what is considered a short shaft. The cav plate needs to be slightly below the bottom of your boat.

 

[guinnessMD]

Unless I screwed up my math, 381 cm is around 150 inches tall.....that would be one really long shaft !!
Now, if you meant 381 mm then yoy woukd need a 15" length or what is considered a short shaft. The cav plate needs to be slightly below the bottom of your boat.

Yes … you right… it is slightly below of the bottom of my boat.

 

[guinnessMD]

 

[guinnessMD]

In my opinion, after half of throttle the engine won’t have the power to push the boat… it is like having it on the wall so it will make water pushing hard and obviously socking air as water goes faster that it is coming in.

 

[guinnessMD]

My model is DF20AS - S stands for 15’

 

[KJM]

Would the problem be the inflatable itself? Inflatables are harder to get up to a similar speed in a boat I've read. Maybe the inflatable is bending slightly up on the front when starting to pick up speed and changing its resistance to the water?

 

[99yam40]

the fact that the motor RPMs go up high at full throttle means the motor has the power to turn the prop, so putting more HP into the prop will solve nothing.
if you know it hits 4400RPMs at half throttle to me is sounds like it maybe too small of a prop.
what RPMs does the motor hit at WOT? does it peg out the tach?
sucking air from under the boat , which should not happen if motor is set deep enough unless there is something under the boat causing disturbed water getting to the prop.
or cavitation is happening.
is it possible your motor is set at the wrong trim
do you have any idea what speeds are involved related to the RPM of the new and old props?

 

[airshot]

My model is DF20AS - S stands for 15’

You mean 15"....15' would be one heck of a long shaft !!

 

[guinnessMD]

You mean 15"....15' would be one heck of a long shaft !!

Yes… 15’’… sorry about that

 

[guinnessMD]

the fact that the motor RPMs go up high at full throttle means the motor has the power to turn the prop, so putting more HP into the prop will solve nothing.
if you know it hits 4400RPMs at half throttle to me is sounds like it maybe too small of a prop.
what RPMs does the motor hit at WOT? does it peg out the tach?
sucking air from under the boat , which should not happen if motor is set deep enough unless there is something under the boat causing disturbed water getting to the prop.
or cavitation is happening.
is it possible your motor is set at the wrong trim
do you have any idea what speeds are involved related to the RPM of the new and old props?

The motors should hit up to 6300 rpm at wot… I posted the specs above … I think I will go on water next week and film everything in order for you to have a better Idea. Thanks a lot for your help in this matter!

 

[flashback]

Weight distribution is a factor as well, if your motor is abruptly over reving. And be sure your boat is inflated properly.

 

[99yam40]

Most regular boats that have power tilt and trim will act like what you are saying if the motor is trimmed up too far under way at higher speeds/RPM.
trim back down a little and all is good again.