Correct prop depth

[Frickinfrack]

Good day all! Not sure if this is the right forum but will ask anyway. I'm in the process of making revisions to a paddle boat conversion to be propelled via modified trolling motor. It has a hull similar to a pontoon boat and I have the trolling motor mounted underneath and in between to pontoons. on high, it sucks enough air that I can hear it and feel the difference in thrust. I know I could just do some trial and error but due to the way it's mounted that would involve drilling more holes in the shaft which I don't want to do any more than needed (see picture). Is there a "hard and fast" rule for how deep the prop needs to be in the water to not suck air? Right now, fully loaded, I've got about 3-4 inches of water over the top of the prop. I suspect the fact that it's in between the pontoons has something to do with the issue.

 

[QBhoy]

I’m almost sure you’re right. That hull will be gulping air from the bow and down between hulls and into the path of the prop. You might be best with it side mounted outboard of one of the hulls.

 

[Scott Danforth]

there is no hard and fast rule

prop depth is dependent on hull and load. with your small hull, waves will also play into it.

keep in mind, your trolling motor does not have an anti-ventilation plate. you may want to simply fabricate one.

 

[Frickinfrack]

Scott Danforth Forgive my ignorance, what is an anti-ventilation plate?

edit: Never mind I decided not to be lazy and google it. Looks pretty simple. Just a shield to direct the water onto the prop and keep the air out, right?

 

[Frickinfrack]

QBhoy That was plan A. It just doesn't fit into the scheme we are going with. Going for a stealth look which it why it is under the boat.

 

[Scott Danforth]

Scott Danforth Forgive my ignorance, what is an anti-ventilation plate?

edit: Never mind I decided not to be lazy and google it. Looks pretty simple. Just a shield to direct the water onto the prop and keep the air out, right?

yes, primarily a plate above the prop to prevent sucking a vortex and ventilating

however you still may ventilate with light people in waves.

 

[GA_Boater]

Trolling motors are designed to operate 12" or more below the water surface. Your prop is barely in the water. Extend the shaft about a foot to put the prop in the water.

 

[Frickinfrack]

Scott Danforth LOL, No worries about light people on this boat. I'm 265 and the wife, well I'm not going down that road. Waves should be very rare as we intend to use it on the Weeki Wachee and Homosassa rivers which is all a no wake zone. I've about got this thing dialed in now. Replaced the old lead acid batterys and relay box with a custom made LiFePo battery pack and PWM speed controller. She now putters right along as long as we are both sitting as far back as we can. The ventilation problem didn't get really bad till I changed batteries and removed about 75 Lbs of lead from the back of the poor thing. I'm thinking of making something out of plastic that has a flat part on top and round bottom to direct the water into the prop. Kind of like a jet drive would have but doesn't extend more than an inch past the prop if at all. That should work, right? Maybe just a flat plate over the prop? I tend to over complicate things, in case you couldn't tell.

 

[Frickinfrack]

GA_Boater I plan on dropping it down a little but because I was a Dumb a$$ and cut the shaft, I can only drop it down about 3-4 more inches. Gonna build a plate or something to keep the water on the prop as Scott Danforth recommended. BTW, Nice vintage Starcraft. I was given a 66 Searay a few months ago and am restoring it. Has a 1984 70 HP Johnson on it. Gets lots of looks!

 

[GA_Boater]

3-4" may not be enough. I'm not sure a plate will help, but you never know.

Or get or make another shaft and don't cut it until you are sure it is deep enough.

 

[Frickinfrack]

GA_Boater I'll try the plate first and if that doesn't work I'll get another shaft. Do you happen to know where I could find an aluminum one for an old foot operated 30 Lb motorguide? All I can find are composite and stainless. If I ever get another hull like this one, I'll probably get an engine mount trolling motor and mount it upside down underneath, thoughts?

 

[Scott Danforth]

bring along a case of beer..... thats 42#

 

[GA_Boater]

Big Box store. The AL shaft ain't nothin' special.

 

[WesNewell]

Before getting a new shaft I'd drop it the 3-4 inches you can now. I think that might be enough. I sure wouldn't drop it a foot unless it's needed to eliminate the problem.

 

[Frickinfrack]

Scott Danforth I don't drink but will bring plenty of food GA_Boater Good deal, thank you. If the plate and/ or lowing the motor as far as I can like it is, I may just get a whole new (used) one with more thrust. I'm thinking 50-70 Lbs. WesNewell My plan is to try the anti-vent plate first then if that doesn't work, I'll lower it as far as I can. If that doesn't help, I'll just get a different, more powerful one. I'd really like to go with a brushless but $$ is an issue there. The main issue is that I set the height with 2 big ole lead acid batteries in the back and now I have 1 LiFePo that weights about 1/4 of that so the stern isn't sitting nearly as deep as it was. This was supposed to be a little quick project to use for the rest of this season. It's gotten slightly out of control but, it's a lot of fun and can go damn near anywhere I've got a foot or so of water. Kinda like a kayak w/o sweating. I've got about $1000 (boat was $525.00) in it so far and don't want to get too crazy as I have 2 other boats to rehab before next summer.

 

[WesNewell]

Don't think an av plate will help at all. just lower the motor as much as you can and try it. Surely the battery weight change didn't affect the height more than an inch or two.

 

[Frickinfrack]

WesNewell It actually make a pretty big difference. More than you would think. Remember we are talking about a 5X8 foot paddle boat (paddle assembly removed) with a capacity of 600Lbs. 75 Lbs make a pretty significant difference considering it was all right on the back of the boat. I plan of doing both actually I have a piece of PVC pipe with a 45 degree elbow to direct the water and will lower it about 5 inches. If that doesn't work, I'll just replace the trolling motor because I already cut the shaft and that is as far down as I can go and still have enough pipe to keep it from flexing/ breaking the mount (1 inch ID pipe and a floor flange screwed through the hull and into a 2X4). Almost hope it doesn't work so I have an excuse to get a bigger motor. It's currently running an old 30 lb motorguide, I'm thinking like a 50 Lb or so would be better. It currently has a 2 blade prop and I have a 3 blade. Might change that out as well. The modification is currently on revision 3. I'm thinking by the time I get to revision 5 or 6 it will be done(ish). Lithium battery pack and PWM has made the biggest difference so far.

 

[WesNewell]

Having been riding and building electric bikes since 2011, I'm pretty familiar with lithium battery packs. I build my own using lipo. My controller on my 4KW rated hub motor draws about 65A with a 24s battery pack charged to 101VDC. My battery pack is rated for 20C continuous max discharge (200A). Trolling motor (MotorGuide also) on my boat draws 35A max iirc. Many lifepo4 battery packs are only rated at 1-3C discharge , so be sure your controller doesn't draw more amps than your life pack is rated for if you change it with a more powerful motor/controller.

 

[Frickinfrack]

I built mine using many 3.75 v cells (3s8p) rated at 10AH each and peak of 50A draw. Also have a BMC that's rated for the same.. I typically pull 15-20 amps headed upstream and 5-7 going back down river. Did lots on investigating on youtube before building. Seems to be working pretty well.

Edit: Just reread, I've (well my wife did anyhow) decided that the 30 Lb is fine and found one on FB marketplace with a bad motor so I have an extra shaft now if needed. Might just replace it anyhow.