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Jan 23rd, 2013
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Lets figure this out.... #14598914 01/16/23 07:30 PM
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flashman252 Offline OP
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Ok need some opinions on this issue. I have a jon boat that I went stupid crazy on. Lets leave the HP out but lets say its 90-115 hp. The boat is rated for a 90 hp, the 115 might be what I went with because the motors are the same weight. The boat is a 1954 Excel aluminum boat. The issue is the boat jumps up on plane. Gets up quickly loaded or empty, but when it comes time to trim out the motor, it starts porpoising. Only way it wont, is the motor stays pretty much buried but I can trim up just a little. This creates quite a bit of tiller torque on the arm and I feel like if I can trim it out, the motor will be WAY easier to run. There is another boat that I have driven with same motor and it wasn't near the bear to run down the lake. Still running 40 mph and I am getting 5200 RPMS, and honestly don't necessarily need more speed, but need to stop the porpoising and be able to trim out the motor on the top end.

Info:
Boat weight - 750 lbs empty. Loaded probably close to 1250 at its heaviest. I know I know, over weight limit but its never seemed like an issue with all the tin rigs we have loaded and hunted out of.
Prop - 19p spitfire stainless prop
Elevation - 4000 ft
Bobs Hydro JP

So we have lifted the motor all the way up on the JP, its new "sweet spot" is 4 on the JP and that allows just the littlest amount of trim up to get higher RPM when on plane. When motor was lower, no trimming out allowed before porpoising started. Again, best we are getting is 5200 RPM, just a touch up of trim on the motor, and anything past this it starts the porpoise.

So here is my dilemma. Props are expensive and I need some more knowledge before another is bought. Surely I want to get closer to max RPMS on the top end and this can be done going down in prop size but doesn't a smaller pitch prop create more lift? Or does a smaller pitch prop create more lift on the hole shot but more slip on the top end and this would actually help with the issue? More slip on the top end would create less bite, more RPM and less lift? Would a 3 blade be better than the 4 blade that is currently on.

I need some prop knowledge here guys. Hoping to receive knowledge from the best out there for this fun little puzzle we just got ourselves in.


Last edited by flashman252; 01/16/23 07:32 PM.
Re: Lets figure this out.... [Re: flashman252] #14599153 01/16/23 11:59 PM
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ssmith Offline
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is it a vee pad hull or a flat bottom if it is a flat bottom it will always porpoise when you try to trim it up that is the way it is.

Re: Lets figure this out.... [Re: ssmith] #14599178 01/17/23 12:24 AM
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flashman252 Offline OP
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Originally Posted by ssmith
is it a vee pad hull or a flat bottom if it is a flat bottom it will always porpoise when you try to trim it up that is the way it is.


Flat bottom. Afraid of that answer honestly. So at this point, chase RPMS and deal with the bounce?

Re: Lets figure this out.... [Re: flashman252] #14599217 01/17/23 12:53 AM
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Pat Goff Online Content
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Hull designs are slaves to physics. Flat bottom boats are designed to well...run flat. Trying to make it do something different will always result in what you've got.

V Pad designs are designed to lift and run with trim angles. That's not what you have.

So, in all of the flat bottom iron hulls I've rigged, once you understand it is what it is, then you try to optimize the total performance. By that you want it to jump on plane quick, have some manners when handling, not be a slide monster, and be able to cross some decent rough water without the prop blowing out and causing problems.

What is needed now is WHAT motor, what your current rpms/speed/characteristics like handling, hole shot etc are. Would probably start with a powertech 4 blade SCD or similar. This prop will calm the bounces down a lot, greatly improve handling and carry a load well. Faster? Probably not, but much better behaved all around.

Your porpoising is caused by your prop losing it's bite at the trim angle you're running, and falls down. Not much fun to drive.


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Re: Lets figure this out.... [Re: flashman252] #14599484 01/17/23 02:39 AM
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Rowdy Offline
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You can bring the jp down and help get the porpoise out of the bow. But over trimming the engine is going to add insult to injury.

That hard right torque steer can be addressed by installing a torque tab on. They really help with that problem. I don't install mine as everyone else because of having to repair many of those that got all jacked up running over rocks and stumps. So we can visit about this later if you would like.

Last edited by Rowdy; 01/17/23 02:46 AM.
Re: Lets figure this out.... [Re: flashman252] #14600502 01/17/23 11:11 PM
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flashman252 Offline OP
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Not necessarily wanting more speed. 40 mph is plenty fast for everything be done in the boat. Less porpoising, less tiller torque, absolutely would be the goal.

40 Mph, 5250 RPMS with a load, 19p spitfire prop, 115 pro xs 4stroke, 4000 ft elevation.

hole shot amazing, runs fast and true up to plane and then once on top end unable to trim out sufficiently to lessen tiller torque. So bear to handle with motor buried, but once adding any trim up, porpoising starts.

So I know I am short on RPMS by 1000. The natural thought is to prop down to achieve higher RPM. My worry here is if I go to a 17p prop, I know this results in better hole shot, but does it result in more lift on the top end. Is lift on the top end causing the porpoising? I don't want to make the issue worse by going down in prop size just to chase RPM's. I know boat hull is not ideal for the set up, but going to try my absolute best to achieve functionality with this setup before throwing around the idea of going to a different hull design all together. I know rule of thumb is 200 RPM per inch of prop, puts me around a 15p. I have also seen calculations to try and figure this out, and based on numbers, I have figured out that a 16p prop is the best outcome. Cannot remember where I saw it but essentially take RPM's achieved and divide by RPM goal. Multiply current pitch prop by percentage, and then subtract number from current prop pitch to correct pitch in prop. For example...

5250 / 6300 = 83.3333%
19 * 0.167 = 3.173
19 - 3.173 = 15.827

Meaning a 16p gets me close to achieving max RPM. Again though, just unsure if propping down helps or hurts porpoising.

Last edited by flashman252; 01/17/23 11:51 PM.
Re: Lets figure this out.... [Re: flashman252] #14600555 01/18/23 12:21 AM
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Blade design, # of blades and prop diameter make a huge difference in lift even in the same pitch. Also aluminum vs. steel


1999 Triton TX21/225 Mercury Optimax
Re: Lets figure this out.... [Re: Pat Goff] #14600597 01/18/23 01:05 AM
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flashman252 Offline OP
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Originally Posted by Pat Goff
Hull designs are slaves to physics. Flat bottom boats are designed to well...run flat. Trying to make it do something different will always result in what you've got.

V Pad designs are designed to lift and run with trim angles. That's not what you have.

So, in all of the flat bottom iron hulls I've rigged, once you understand it is what it is, then you try to optimize the total performance. By that you want it to jump on plane quick, have some manners when handling, not be a slide monster, and be able to cross some decent rough water without the prop blowing out and causing problems.

What is needed now is WHAT motor, what your current rpms/speed/characteristics like handling, hole shot etc are. Would probably start with a powertech 4 blade SCD or similar. This prop will calm the bounces down a lot, greatly improve handling and carry a load well. Faster? Probably not, but much better behaved all around.

Your porpoising is caused by your prop losing it's bite at the trim angle you're running, and falls down. Not much fun to drive.


Patt saw your other post in the off topic section. Replied here before going over there and looking. Most definitely will be looking at the SCD4 prop. Sounds like this would be the prop to go with to try and cut down porposing. Honestly, if I could trim out the motor and get the porposing to stop, I would be estatic. Even if I gave up a couple mph on the top end. 35-40 is plenty fast for everything we do. 40-45 would be glorious, but I understand can't have it both ways. So propping down to a 16p, gonna run a prop specifically designed to cut down on porpoising and see how this goes.

Re: Lets figure this out.... [Re: flashman252] #14600714 01/18/23 04:05 AM
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Originally Posted by flashman252
Originally Posted by Pat Goff
Hull designs are slaves to physics. Flat bottom boats are designed to well...run flat. Trying to make it do something different will always result in what you've got.

V Pad designs are designed to lift and run with trim angles. That's not what you have.

So, in all of the flat bottom iron hulls I've rigged, once you understand it is what it is, then you try to optimize the total performance. By that you want it to jump on plane quick, have some manners when handling, not be a slide monster, and be able to cross some decent rough water without the prop blowing out and causing problems.

What is needed now is WHAT motor, what your current rpms/speed/characteristics like handling, hole shot etc are. Would probably start with a powertech 4 blade SCD or similar. This prop will calm the bounces down a lot, greatly improve handling and carry a load well. Faster? Probably not, but much better behaved all around.

Your porpoising is caused by your prop losing it's bite at the trim angle you're running, and falls down. Not much fun to drive.


Patt saw your other post in the off topic section. Replied here before going over there and looking. Most definitely will be looking at the SCD4 prop. Sounds like this would be the prop to go with to try and cut down porposing. Honestly, if I could trim out the motor and get the porposing to stop, I would be estatic. Even if I gave up a couple mph on the top end. 35-40 is plenty fast for everything we do. 40-45 would be glorious, but I understand can't have it both ways. So propping down to a 16p, gonna run a prop specifically designed to cut down on porpoising and see how this goes.


I would HIGHLY suggest you go straight to a professional. I've rigged a lot of boats, but I've not rigged many four strokes. I'd call Josh at Steves prop on fork and ask him what to sell you. Internet advice is sketchy at best, can't see what you've got, can't feel what you feel. They handle the PT prop, or he might have something better that I'm not aware of, but it will save you a lot of time and money to call first, they will do a swap out program if needed, but usually it's not.


Pat Goff
Seadrift TX
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