Straight Shaft Motorized Kayak

Hello everyone. Brand new member.
I have been using a motorized yak for fishing about 15 years. In fact, I built the first motorized yak I ever saw. 1st one was a 12 ft with a 30 lb thrust cheapo mounted on a 2x4 sticking out the side. Worked pretty well actually with top continuous speed of about 4 mph.
Trying to get more speed and room, I built a 15 ft with a 12v, 55 lb motor, stern mounted on a home made jack plate with foot cable steering. For my trouble (and expense) I gained all of .5 mph.
Next, I went to a 24v, 86 lb mounted the same way. I gained all of 2 mph. That was better but not what I expected. I used this set-up for the last 5 years but now the bearings are out on the motor.
I am thinking about going to a 12v, 2000 watt, 150 amp@2700 rpm full load, 3 hp, dc motor, internally mounted and coupled to an 18", 5/8" diameter straight shaft supported by 2 pillow block bearings and a 3" brass bushing threaded thru the stern as a shaft tube.
My question is this....What calculations do I need to do to have a reasonable expectation of 10 mph or better? The yak is 15' at the water line and about 350 lbs loaded with me, my gear and two 120 ah batteries.

 

10 mph is in the planing range for that waterline length. You would need at least 10KW to get close, assuming the hull shape is conducive to planing. The range will be very limited with those batteries.

 

Was thinking about a light weight foil to achieve planing but I am not really interested in jamming a log or some other floating object through a plastic hull at those speeds. Not to mention the handling nightmare of being on plane using a rudder for steering. If I have to settle for 8-9 mph, I can live with that. Any less and I will just go with another trolling motor. The 24 v setup drew 28 amps constant at full power and I averaged 6 mph in calm water with no current. That gave me about 30 mile range which is quite sufficient.
Do you know the waterline length, overall weight and power formula to determine approx max hull speed ? The formula I found says the hull speed of 15 ft is less than 6 mph. I achieved close to that with a 12 volt trolling motor. I guess there is no way to tell if adding the hp will push me up another 2-3 mph except to do it and then experiment with different props and weight dist.
Any thoughts?

 

The formula is the square root of the waterline length multiplied by 1.3 to 1.5 depending on the shape. (15^0.5)*1.4=5.42 knots. The power required to go faster increases exponentially.

 

Thanks...That's what I got also. I think the hp and torque increase ( from 34 inch lbs on the 24 volt to over 80 inch lbs at running loads of 2k watts, the 3 hp will push me faster than hull speed if I can keep the bow down without diving.
But now I watched the video of an electric 'surface drive' posted by Irie and it got my attention. This just never stops does it?

 

Na, it never stops

 

Your surface drive boat has me intrigued. How long is the drive shaft and how many bearings do you use to stabilize it inside the sleeve? How did you couple it to the motor shaft? I was experimenting with an old 1 hp, 12 volt winch motor trying to achieve some sort of semi surface drive long tail and never got past overcoming the shaft wobble vibrating everything apart. I am not very skilled in mechanical design obviously.
The shallow water applications of your surface drive would be perfect for my kayak., I am only trying to achieve 10 mph or so without getting up on plane and it appears that your surface drive design might function better than the straight shaft submerged prop design I was thinking about in my above post.
What RPMs are you turning at the prop when at full speed? How much does your boat weigh fully loaded? Any help would be greatly appreciated.

 

To attain speeds of 10 mph on a short waterline the only option is to make it plane. It is a matter of physics.

 

Thanks for taking an interest in my drive. Its basically an inboard short tail. Im no expert, but I am happy to share what I've done.

My drive shaft is 3/4" rod 36" long. I have a cutlass bearing at the prop end of the sleeve and I use a thrust bearing inside the coupling housing before the shaft couples to the motor. For the coupler I'm using nova jaws.

With the 9.25*10 propeller the prop turns 2700 rpm and the boat does 19-21mph. I believe it weighs in around 600+ lbs with 1 person. But I've never weighed it.

How long was the shaft on your experimental drive? What were you using for a propeller?

 

My Kayak with 80lbs-12V brushless trolling motor.
Henry Zhou https://www.facebook.com/henry.zhou.395/posts/322941591459024

 

Appreciate the info. I was using a 3/8" stainless rod 26" long with a 3" brass bushing thru hull for prop support. The prop was a 8" 3 blade.
It was an old winch motor so it didn't have a shaft, instead it had a gear. I fit a star socket onto the small gear with a nut welded on the end to thread the shaft onto. Anyway, the harmonics were horrible with the thin shaft not being exactly balanced and the thing vibrated intolerably. It did hold together long enough for me to know that a 1 hp motor won't provide anywhere near the power I need.
My set up will weigh about half of yours. So I'm thinking a 4.5 hp, 24 volt, 6k watt motor that turns about 3k rpm under full load will do the job that I want. I also will go to a 5/8 inch shaft supported by 2 pillow block bearings and a 3" cutlass at the prop end. There is no way I will be able to pivot the motor and shaft like yours for steering so I will design a foot controlled rudder that directs the thrust behind the prop. I may also have to widen the stern underneath with some sort of angled plate(s) if planning is a must to attain the speeds I want.
What is the HP rating rating of the motor you use? Does your shaft exit the hull below the waterline?

 

I'm using a 10kw continuous, 19kw peak brushed dc motor (motenegry me1004). 11kw pushes the boat 21mph with the drive running at the surface. I get slightly slower speeds running the drive submerged at 20° but it also requires more power.

My shaft exits about 4" from the bottom of the transom. It is usually just below the water line at rest. I've not had issues with water leaking thru the shaft.

I am not sure that cutlass bearings are made for 5/8" shafts. I feel like 3/4" was the smallest I could find. Also, how much unsupported shaft did you have extending from the bushing to the propeller?

Do you have pictures of your kayak? I suspect it would be difficult to do a steerable drive, but I strongly recommend that the drive be able to trim. As trim and propeller cupping were essential to my drive going faster than 8mph.

One option would be to mount the drive on top of the kayak like a traditional long tail would be mounted. That would give you better capabilities than a trolling motor I'd think. You would still be limited by hull design but someone might be able to help you convert to a planning kayak, maybe? That's over my head, but I'd be interested to see it.

 

I am looking at the motor you are using. With controller it's over a grand and produces about 12 hp at working loads. Impressive. I need to be reasonably certain of the results before I even think of spending that kind of money. I found a high torque 4.5 hp that runs on 12 or 24 v...It produces over 70 inch lbs torque at 24 v using 4.5 kw running over 3k rpms, 170 amps. The specs say at 12 volts it will produce about 2100 rpm@ 140 amps constant. It has a 3/4" shaft so I will go to a 3/4"as well. Overall cost will be about $300.00.
I can square the stern planning surface with a trim plate if I have to. Light weight aluminum should work. I see those ganoes easily get up on plane with small gas motors and the stern on those isn't much wider than mine. The steering will be a big problem if the idea of a foot controlled rudder doesn't work. I am really hoping it does because everything goes back to the drawing board if it doesn't.
How sharp can your boat turn?

 

The drive turns roughly 25-28° which is acceptable for my use but could be improved. Some of my videos show 180° turns if you wanted to actually see it turn.

I don't see why a rudder wouldn't work, or did you mean the foot control aspect?

 

Thank you for all your help. I have decided to go with a top mount semi long tail set up. The motor (4 hp ), coupling and about 6" of shaft will be inside an aluminum housing secured to a flange mounted ball joint. That will provide the up/ down side to side motion I need for trim and steering. The shaft will exit the housing through a flange bearing bolted to the inside of the housing. I will have to find some sturdy aluminum pipe that will fit over the shaft and also fit a mount to the outside of the housing. I will also have to find a cutlass bearing that will fit on the drive shaft and into the sleeve. The shaft will exit the housing only a few inches from the stern and will be only about 5" from the water so a 40" shaft will be long enough to keep the prop at less than a 20° downward trim even at full submersion.
There is one dilemma though. I can't attach an actuator for up/down (trim) because the entire housing has to turn for steering. Any ideas?