Same RPM, same speed with 17 and 22 inch props.

[Ram_Boat]

Signature Cleaver 22 in
vs
Mercury Black Max
same boat, same RPM same speed.

I'm trying to understand what is going on.

I have tried different eights using jack plate from way too low to too high. With and without an hydrofoil. The best speed was with the smallest prop 4 800 RPM @ 32 mph using gps. I loose about 2 mph with larger prop.

[Ram_Boat]

Now with 11 inch pitch prop I got 5 500 rpm @ 28 mph (not full trottle). I think I may reach 6 000 but did not dare to keep there long enough to read the gps. Yes acceleration is faster with such low pitch but there is spare rpm to get faster. I guess my next step is the get the 11 pitch prop repitched to 13and the 17 inch repitch to 15 and test what is the best.
Am I on the right track to think the drag increase the prop slip? This would explain why I get same rpm and same speed with 17 and 22 inch props. Also guessing I would end up in the range of 35 mph?

[eli]

pics would help of motor area in relation to back of boat

[Ram_Boat]

I will post pics tomorrow on the trailer for the moment this is all I have. I find there is a lot of water being shooted upward. All around the motor even in the bracket and back in the boat.

Here is a 2 second video @ 28 mph.


On Sept 6th 2014. Before highjackers were modified
https://www.youtube.com/watch?v=VCls9ug0kMU
On Oct 19th After highjackers now have about 5 inches of flat surface
https://www.youtube.com/watch?v=QZQqf1nAzGA

[wettek]

Try lifting your engine, from the amount of spray, it looks way too low. Bolts should be in (looking at the amount of spray in your picture) 2nd hole from the bottom, if not the bottom.

[eli]

shaft length ??

[Ram_Boat]

Long shaft.... found too late I should be using short shaft.
The current pick is about to the lowest position on the jack plate. I can raise up to 5 inch.

[eli]

that's it you found problem

[Ram_Boat]

I tested today with engine raised near it's maximum. The problem of spray all around is not hapening any more. I also added a metal plate to close the bottom part of my custom made jack plate. Still a bit of water in the boat from the motor braket. For the moment this not a major concern.
The engine was too high I was barely able to take off. Once on plane the prop would loose it's grip in any waves higher than 10 inch . I will try tomorrow with engine 2 inch lower than today.
The pic below show the larger section of the lower unit that was raise just above the floor. Is it possible that this section was causing all the spray?

[Capt.Insane-o]

That boat will never run with an aluminum prop.

[Ram_Boat]

Thanks for the feedback Capt.Insane. I will keep this in mind.
I tested tonight with engine a bit lower than my last post and the spray problem is still under control and prop loosing it's grip is 80% solved. Check pick below, was taken @ 33 mph cavitation plate is about 1/2 inch above water I will try to lower the motor 1/2. Looks like the ideal eight will be within an inch from my current position. I may get 1 - 2 mph more but still far from what I was expecting from this hull.


This bring back to the original question that started this post. How come props of 17 and 22 at same rpm push the boat at the same speed? Could it be the bottom part of the hijackers are round as visible in the pic. of my post dated of 08-23-2014, 11:00 AM. Missing flat surface = not enough surface to plane on them. I'm currently running on the tubes so the hijackers are all the way in the water. Could it be the cause of additional drag that would explain why the 22 inch prop drive the boat at the same speed than the 17 inch prop.

[Capt.Insane-o]

Do some research on the props the super lights use. http://www.apba.org/slt

[milkdud]

Capt.Insane-o
Team Member

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That boat will never run with an aluminum prop.

AGREED

[Ram_Boat]

I agree with you that stainless steel would be better. Question : how much gain should I expect from my current 32 mph speed with a stainless prop?

[Ram_Boat]

I'm about to order a new prop; Merc Vengeance 15''based on the following:
The Blackmax 17'' seems to be too much pitch with rpm@ 4800 and speed 32 mph.
Hope to be able to go across waves without loosing all prop grip + gain a few more mph.
Is there a better alternative as for prop model?
Is the 15" pitch a good choice?
Should I go with 10 or 10.25" diameter?