More prop or lower motor?

[leckbass]

Lot's more info needed here.....motor height..engine rpms at wot, rpms getting on plane, just to start. Here is how to get your motor height ....Borrowed from the Champion Boats Forum.

18 ft Fisher SV 18 CC Aluminum, 2000 Mercury 125hp, boat weighs approx 700lbs, motor is approx 350lbs, Minn Kota Riptide at the bow 100lbs, 2 50ah lithium batteries in the front 25lbs, 1 group 31 battery in the back 75lbs, 18 gallons of gas in the back 108lbs, approx 500lbs of passengers, 1858 lbs total. The motor has a dol-fin on the back.

PTP is 8in
Turning Point Hustler 13x19 4 blade 35.5 mph @5250 RPMs, boat is on plane at approx 2500RPMs
Mercury prop 12.75 by 21 3 blade 36mph @ 5250 RPMs, boat is on plane at approx 2500 RPMs.

I don't push the motor past 5250 because that's the maximum RPM range.

With both props, when I'm running at full speed, I can begin to trim up and the boat starts getting faster 37, 38, 39... but the boat begins to porpoise and I have to trim back down.

[leckbass]

LOL You're wanting a yes or no answer to an essay question.
The info you posted in the setups forum doesn't even apply anymore because it was a different engine. About all anyone could tell you with the information you've given is try it and see what happens.

18 ft Fisher SV 18 CC Aluminum, 2000 Mercury 125hp, boat weighs approx 700lbs, motor is approx 350lbs, Minn Kota Riptide at the bow 100lbs, 2 50ah lithium batteries in the front 25lbs, 1 group 31 battery in the back 75lbs, 18 gallons of gas in the back 108lbs, approx 500lbs of passengers, 1858 lbs total. The motor has a dol-fin on the back.

PTP is 8in
Turning Point Hustler 13x19 4 blade 35.5 mph @5250 RPMs, boat is on plane at approx 2500RPMs
Mercury prop 12.75 by 21 3 blade 36mph @ 5250 RPMs, boat is on plane at approx 2500 RPMs.

I don't push the motor past 5250 because that's the maximum RPM range.

With both props, when I'm running at full speed, I can begin to trim up and the boat starts getting faster 37, 38, 39... but the boat begins to porpoise and I have to trim back down.

[FES313]

I had that motor on a Ranger R71. Ran a 23 Lazer II in the upper 50s. I'm gonna vote more prop.

[BassinPaPaw]

I had that motor on a Ranger R71. Ran a 23 Lazer II in the upper 50s. I'm gonna vote more prop.

I’m by no means an expert on this, but this is my thought as well.

[XingEyeballs]

If you’re 8” PTP then I’d be leaning towards bigger diameter and probably more pitch too

[leckbass]

I had that motor on a Ranger R71. Ran a 23 Lazer II in the upper 50s. I'm gonna vote more prop.

I checked another manual and it still said 4750 to 5250, what did you run yours at? Max rpms?

[cubswin]

I'd see if you could run a ss prop over the aluminum, and go to probably a 22p. I'd check with a local repair shop and see if they have one you can borrow to test with.

[bassfisher444]

An aluminium prop is one problem, you need a stainless prop to get good perfomance out of that setup.

[angleiron]

If you are only turning 5250 then more prop is the LAST thing you need. Same for lowering the engine as that is going to reduce rpm's. Do you need a Dol-fin to be able to get the boat on plane, and is this out of the water when you are running? Porpoising is usually a weight distribution problem not a speed problem. Aluminum prop could be part of the problem. Are you running a large hub prop or a small hub prop (exhaust through the hub *large hub prop* or exhaust through and over the hub*small hub prop*)?

[leckbass]

If you are only turning 5250 then more prop is the LAST thing you need. Same for lowering the engine as that is going to reduce rpm's. Do you need a Dol-fin to be able to get the boat on plane, and is this out of the water when you are running? Porpoising is usually a weight distribution problem not a speed problem. Aluminum prop could be part of the problem. Are you running a large hub prop or a small hub prop (exhaust through the hub *large hub prop* or exhaust through and over the hub*small hub prop*)?

5250 is the max RPM, I do not need the dolphin at all to get on plane, I just added it to try to solve the porposing problem, I can easily remove it again. I can see the dolphin when I'm on plane. All of the props I have run are standard size for that motor, the exhaust is through the Hub

[rednecktech]

If you are only turning 5250 then more prop is the LAST thing you need. Same for lowering the engine as that is going to reduce rpm's. Do you need a Dol-fin to be able to get the boat on plane, and is this out of the water when you are running? Porpoising is usually a weight distribution problem not a speed problem. Aluminum prop could be part of the problem. Are you running a large hub prop or a small hub prop (exhaust through the hub *large hub prop* or exhaust through and over the hub*small hub prop*)?

He quoted earlier he reaches max rpm, but he is not at WOT, which would seem to point to the need for a bigger prop. Weight distribution could also be an issue as well with the TM batteries up front.

[leckbass]

He quoted earlier he reaches max rpm, but he is not at WOT, which would seem to point to the need for a bigger prop. Weight distribution could also be an issue as well with the TM batteries up front.

I used to have two group 31s up front that weighed about 75 lb a piece, I replaced them with two lithium batteries that only weigh about 12.5 each, just a Riptide up front

[novakevlar]

An aluminium prop is one problem, you need a stainless prop to get good perfomance out of that setup.

Second problem, Turning Point props are regarded by many to be garbage.

I think the diameter might be the issue here as well. Once that boat gets towards WOT speeds, the resistance that the prop is encountering from the wetted surface needs to be countered with "grip" - if the prop doesn't have enough blade surface area (usually dictated by diameter) it will lose the battle of resistance and the bow will fall and speed will drop. Then the prop re-grips, it climbs in speed again until the resistance overcomes the prop again and it drops. Wash rinse repeat.

Center of gravity is probably a small factor as well. Engine mounting height as well. This is why it's not always a black and white answer, dialing in a prop and boat is a lot of trial and error.

OP get a better Yamaha or Merc prop, get a little more diameter and see what happens. If you stick with aluminum it won't be as pricey, or if you go SS then guys like Ken at Propgods might still do a prop swap deal.

[FES313]

I checked another manual and it still said 4750 to 5250, what did you run yours at? Max rpms?

I don't quite remember but it would bounce the rev limiter once in a while when I was by myself. I had 2 speeds back then, wide open and off.

[The5thDutton]

Some cup in the blades will help with grip.

And for a non-pad bottom boat, I've had best luck with stern lifting props. The bow lifters tend to induce porpoising when there's no pad to ride on.

[angleiron]

He quoted earlier he reaches max rpm, but he is not at WOT, which would seem to point to the need for a bigger prop. Weight distribution could also be an issue as well with the TM batteries up front.

If his max rpm is 5250 then going to a higher pitched prop is going to lower those rpm's even more...so will lowering the engine as that is more drag. He needs to go to a lower pitched prop to gain the rpm's which also means a little more HP gained as he will be able to reach the rpm's where there is more HP available. If he has batteries in the bow that is where the porpoising is coming from when running as he is probably over trimming the motor to try to get the rpm's and speed.

I have had a tin boat before which was a 17' with a 90 etec that turned a 19 pitch prop at would do 45mph but it was not your "usual" tin boat design. It was a Sea Ark that was also built in collaboration with Allison so it was built with a little performance in mind . My boat was designed for batteries in the front but I moved them to the back as the boat would porpoise when they were in the front. I put a $200 manual jackplate on my tin boat which had a 6" setback so I could move the engine height up/down easier.

If higher rpm's are needed he will need to go to a lower pitch prop. Each drop in pitch is 150-200 rpm's gained so you do the math. If the dol-fin is not needed then get rid of it. I would try to move the batteries to the back of the boat so there is bow lift which means less hull in the water also gains rpm's. I would move the batteries first then see what happens as far as rpm's.

[unknown angler]

8 inches prop to pad seems quite deep, I don't think the aluminum prop is helping the situation either. A 21 pitch Laser II should get you close if you raise the engine, how much to raise it will be trial and error.

[Rulebender]

If his max rpm is 5250 then going to a higher pitched prop is going to lower those rpm's even more....

He is not hitting max rpm, he is self limiting himself to 5250 rpm. He has more throttle that he can give the motor but he does not as to not go over the max reccomended rpm. Which suggests he need a bigger prop that will max out at the proper max rpm zone. More pitch and more diameter most likely so that when at WOT he is near or at max rpm range.

[leckbass]

If his max rpm is 5250 then going to a higher pitched prop is going to lower those rpm's even more...so will lowering the engine as that is more drag. He needs to go to a lower pitched prop to gain the rpm's which also means a little more HP gained as he will be able to reach the rpm's where there is more HP available. If he has batteries in the bow that is where the porpoising is coming from when running as he is probably over trimming the motor to try to get the rpm's and speed.

I have had a tin boat before which was a 17' with a 90 etec that turned a 19 pitch prop at would do 45mph but it was not your "usual" tin boat design. It was a Sea Ark that was also built in collaboration with Allison so it was built with a little performance in mind . My boat was designed for batteries in the front but I moved them to the back as the boat would porpoise when they were in the front. I put a $200 manual jackplate on my tin boat which had a 6" setback so I could move the engine height up/down easier.

If higher rpm's are needed he will need to go to a lower pitch prop. Each drop in pitch is 150-200 rpm's gained so you do the math. If the dol-fin is not needed then get rid of it. I would try to move the batteries to the back of the boat so there is bow lift which means less hull in the water also gains rpm's. I would move the batteries first then see what happens as far as rpm's.

Just to clarify, the engine reaches its Max recommended RPMs which is 5,250 RPM, it will spin the props faster but I do not want to force the engine passed it's Max RPM, also the TM batteries are 12.5lb lithiums

[leckbass]

8 inches prop to pad seems quite deep, I don't think the aluminum prop is helping the situation either. A 21 pitch Laser II should get you close if you raise the engine, how much to raise it will be trial and error.

The engine is raised as high as it'll go unless I install a Jack Plate.