Posts: 3472
Location: Elk River, Minnesota | Hiya!!
Before going too far, my suspicion is that we have an over-pitched prop on the boat. If this is a 4 stroke 60hp motor, it has a low gear ratio (current is 2.29:1) which is a great thing for low end torque. RPM specifications list 5300 to 6300 at wide open throttle, so with running 5200 on your rig as it stands, I am thinking you are over-pitched by about 4 inches.
The boat itself is close to 1000 pounds, so getting 31 mph is honestly quite good...but...if the specs have not changed from when you purchased to todays model, the over-pitch of the prop is what is definitely causing your issues.
When a motor is over-pitched, it has a tendency to porpoise badly when trying to trim up because there is so much downward force at the rear, but not enough RPM's to keep the bow up...thus, the bow rises and falls because of the lack of rpms. So...going down in pitch will allow the motor to reach higher rpms, and allow the motor to turn enough to keep the bow up while trimming.
At 60hp, a steel prop will not give much of a performance gain, but it will give you some durability and less flex. I'd stay stay with aluminum, but go to a good aftermarket like a solas propeller. They used to carry what they called the allcup series, but I think that has been updated to the Amita Series. These are a good style of prop as their casting is a bit different than a normal aluminum...they are squeeze cast, which means they are under more pressure in the casting process, thus allowing them to make a thinner blade. You get the best of both steel and aluminum...less flex in the blade like steel, but yet the price and softness of aluminum if you hit something. Granted, at full throttle, the prop metal will not make a difference...damage will happen...but for both durability and performance, it is tough to go wrong with solas.
I'd highly consider going to an 11 pitch propeller, which would gain you about 800 RPM if memory serves me correctly. going up 800 rpm would give you the rpms you need at full throttle to maximize the efficiency of the engine. Being the boat is heavy, I'd even consider going 4 blade, which would allow you to move the motor up a hole or two to maintain your rpms.
I'm one to feel that the only boats requiring a fin are those that are underpowered...in your case for a tiller model, the 60 is max hp, so adding a fin to that is only disguising the issue, and you would still be lugging the motor. When rigged correctly, the fin would not even be in the water at wide open throttle.
My gut says...go to an 11 pitch prop 3 blade, then move the motor up one hole. If you go 4 blade, go up 2 holes. In the end, I think you will feel like your boat will wake up and really be a nice rig to run...
Steve |
