I would like more top end speed

[SNye45]

I have a 20ft Aluminum Thunderjet boat with a Suzuki 115hp OB. Because I live and operate the boat at 3,000 ft elevation the dealer swapped out the standard 21" pitch prop to that of a 19" pitch. I top out with a regular load of myself, my wife and our dog @ 36 mph doing 5,300 rpm.

My next door neighbor has a similar 20ft Aluminum North River boat with the same Suzuki 115hp OB. He tells me that he tops out @ 41mph with a load of 4 people. When I asked him what pitch his prop is -he did not know -he has never changed it since buying his boat.

Question: Would I gain the extra 5mph by trying to go back to a 21" pitch or will that "damage" my engine (operating range 5,000-6,000 rpm) and adversely affect my existing hole shot. I would rather have a higher top end, than a better hole shot. Or should I just be happy with what I have already?

Thank you

 

[Bifflefan]

Re: I would like more top end speed

if 5300 rpm is WOT then you wont gain any speed with a 21p prop.You will lose rpms and maybe keep the same speed, also lose some hole shot.

If its not WOT then you can try it. It may or may not help. If you gain its gonna be 2 MPH at most.

The best advise is, if you want a faster boat, buy a faster boat.

 

[steelespike]

Re: I would like more top end speed

Are these speeds by gps?If not the difference may just be speedo error.
If they are not identical boats brand,model and year there could be a big difference in design and speed.Prop design and material could make a difference.
If his setup is carefully done and yours is not so much that could account for some difference.Are the motors 2 stroke or 4 stroke. I would shoot for more rpm closer to 6,000.You might do a compression check,be sure the throttle opens all the way.Maybe a tune up, check timing.Prop in excellent condition.
Motor setup? Raise the motor a little at a time to a point just before venting(cavitation) becomes a problem.Most end up with the antivent(cavitation) plate about an inch above the bottom.

 

[SNye45]

Re: I would like more top end speed

Thank you for your replies.

My speed is by indicated by my GPS. He has a dedicated speedometer. The engines are identical being a 4 stroke, fuel injected 115hp. Our boats are very similar, made out of the same gauge aluminum, same length etc. His might be a tad heavier as he has a wakeboard tower. I hadn't thought about instrument discrepancy.

This is at WOT and I try and trim it out just below porpoising and the prop slipping.

 

[skibrain]

Re: I would like more top end speed

As another said, I would recommend comparing the height the motors are bolted to the transom. This is checked with the boat on the trailer, motor lowered to the down position and then trim adjusted back up to where the cavitation plate above the prop is parallel with the bottom of the hull. (A yard stick or straight 1x2 laid along the bottom of the hull or along the cav. plate can help you sight this. It takes a little bit of fiddling with the trim.

A standard mount height for many boats has the cavitation plate even with the lowest part of the hull. More lower unit in the water creates more drag, keeps the prop in the water more securely in tight turns etc. Less lower unit in the water creates less drag and lets the motor turn more RPMs and possibly benefit from a higher pitched prop. I have had a 15' and 16' sport boat respond 3-4 mph by raising the motor just 3/4". Other boats may not be that sensitive to this adjustment. The down side of getting too high is that you might get ventilation at hole shot or in turns.

You will notice that on your outboard, the bracket that mounts fixed to the transom, has a series of holes, or a long long slot with a series of round holes about every 3/4". This allows for adjustment without drilling any new holes in the transom.

Assuming that your steering gear and controls give clearance for a change in position, it is fairly easy to do. I've used a scissors jack and a block of wood under the skeg of the motor to lift (or lower it). Loosen (DO NOT REMOVE! unless you have the equipment or friends to pick a 400lb motor off the driveway!) the transom bolts allowing the motor to slide. I wouldn't try adjusting more than one hole at a time either way. Tighten and test. Compare RPM at WOT and see what you find.

If you run a higher motor height you will likely benefit from a stainless steel prop with a cupped edge on the blade. This type of prop is more efficient at holding the water when running closer to the surface.

 

[180Fisherman]

Re: I would like more top end speed

See if you can borrow his prop. No harm no foul.

 

[Silvertip]

Re: I would like more top end speed

First, there is no such thing as a "standard" prop. In fact engines don't even come with them since the manufacturer has no idea whether that engine is being installed on a barge, pontoon, catamaran, or a runabout. The dealer installs whatever he/she feels is appropriate for that area or what the new owner "needs". That assumption is rarely spot on. 5300 rpm means you are over propped (too much pitch), engine mounting height is wrong (too low), the throttle lever is not opening the throttle fully, the engine is simply not making full power or you have an extremely fouled hull. What may appear as similar boats in practice may not translate to even close performance numbers. That is simply not a way to measure what your boat is capable of. The best prop is one that allows the engine to rev at or very near the upper end of the manufacturers rpm band at wide open throttle with what you consider an average load. Top speed will be whatever that prop provides. You do not approach this goal in reverse -- as in saying I want to achieve 41 MPH what prop do I need. The fact is that your boat may not be capable of 41 MPH regardless of prop. Did your boat ever hit 41 MPH at a lower elevation.

 

[45Auto]

Re: I would like more top end speed

My speed is by indicated by my GPS. He has a dedicated speedometer.

Speedometers are NOTORIOUSLY inaccurate and tend to read high. Take a ride on his boat and bring your GPS with you ........