Dingy Motor Shaft Length

[FWT]

Pretty sure I know the answer to this, but I thought I'd open the question to the collective wisdom.

I have narrowed down dingy selection to 2 brands (Whaly, and Polycraft). I have not yet been able to find a weekend to go compare and decide. And, its early to be ordering a dink when the mothership on order won't arrive until next spring.

But I know I will be using an ePropulsion motor. Not looking to plane the boat.

One (whaley) specs a short shaft. The other (Polycraft) specs extra short.

I had been waiting to order the motor until I had nailed down the dink selection. Lithium is getting in shorter supply, as are circuit boards. It dawned on me this morning that delay might not be necessary.

There is a 4 inch difference in shaft length between short and extra short.

Other than the obvious bottom drag when beaching, at slower speeds does a shaft 4 inches longer than perfect make a material difference?

It dawned on me that Torqeedo offers short, but not extra short. If that's what I wanted I'd probably be inclined to just get short and shrug the shoulders. Why not just get a short shaft ePropulsion (not extra short) for use on either choice?

What is the flaw in that thinking?

I've pretty much ruled out a Pudgy on stability reasons. But it takes a long shaft. Maybe some other dink option comes into view that needs a long shaft. Seems to me the biggest reason to wait is to just be dead sure on dink selection, and not that a shaft 4 inches too long presents a material issue. The biggest reason to go ahead and order a motor is just supply chain concerns.

Thoughts?

 

[DennisB1]

The prop should below the boat bottom to have clean flow. The unit with the extra short shaft does not provide much freeboard if it mounts like a typical outboard. I would be worried about flooding the dinghy with a few people in the boat

 

[FWT]

The prop should below the boat bottom to have clean flow. The unit with the extra short shaft does not provide much freeboard if it mounts like a typical outboard. I would be worried about flooding the dinghy with a few people in the boat

The question is more about having a shaft longer than spec, not too short.

And FWIW here is the dink that specs an extra short shaft.

https://www.polycraft.com/300-tuffy

Capacity 3 people

 

[Nancy n Larry]

We have a Whaly 310 with a 9.9 Tohatsu, short shaft. Love it, though we may need to get shark fins for the motor. The boat can hold a lot of stuff and is quite roomy. Also very tough.

 

[backinblue]

I bought an eProp a couple years ago and went with the dealer recomendation and bought the short shaft w/o doing any measurements or research. I love the motor but really wish I went with the xtra short. The eProp website has clear drawings that help you select based on your transom height. Having the extra length makes it more difficult when beaching. FYI you will never plane with this motor so that's not a consideration, it's not designed for that. It will run for many hours at reduced throttle, but full throttle doesn't add much speed, just pushes more water. My biggest regret is not paying attention to the specs and ending up with a longer shaft than needed. Note that eProp sells the motor and battery separately. If lithium supply is becoming an issue, you could order the battery now and order the motor later.

 

[FWT]

We have a Whaly 310 with a 9.9 Tohatsu, short shaft. Love it, though we may need to get shark fins for the motor. The boat can hold a lot of stuff and is quite roomy. Also very tough.

Thanks for this. I do see positive comments from users. And nothing negative other than the standard stuff relating to the fact its not the standard RIB design.

My #1 reason to go in this direction is stability when boarding. Your experience with that aspect?

 

[FWT]

I bought an eProp a couple years ago and went with the dealer recomendation and bought the short shaft w/o doing any measurements or research. I love the motor but really wish I went with the xtra short. The eProp website has clear drawings that help you select based on your transom height. Having the extra length makes it more difficult when beaching. FYI you will never plane with this motor so that's not a consideration, it's not designed for that. It will run for many hours at reduced throttle, but full throttle doesn't add much speed, just pushes more water. My biggest regret is not paying attention to the specs and ending up with a longer shaft than needed. Note that eProp sells the motor and battery separately. If lithium supply is becoming an issue, you could order the battery now and order the motor later.

Your regret is just beaching?

Yeah, I know that motor isn't for speed. Willing to give that up in exchange for simplicity to mount it, store it, feed it, maintain it, and ease of operation for my non-mechanical wife.

Clearly the best path is pick the dink then the motor.

 

[High Wire]

Extra short? Never heard of it.
Short = 15”
Long = 20”
XLong = 25”

 

[FWT]

Extra short? Never heard of it.
Short = 15”
Long = 20”
XLong = 25”

SPIRIT 1.0 Plus Tiller – Extra Shortshaft (20.7"), SPIRIT 1.0 Plus Tiller – Short Shaft (24.6"), SPIRIT 1.0 Plus Tiller – Long Shaft (29.5"), SPIRIT 1.0 Plus Remote – Short Shaft (24.6"), SPIRIT 1.0 Plus Remote – Long Shaft (29.5")

 

[FWT]

Extra short? Never heard of it.
Short = 15”
Long = 20”
XLong = 25”

Torqueedo:

Shaft length: short 62.5 cm / long 75 cm

(24 inches / 29.5)

 

[backinblue]

Your regret is just beaching?

Yeah, I know that motor isn't for speed. Willing to give that up in exchange for simplicity to mount it, store it, feed it, maintain it, and ease of operation for my non-mechanical wife.

Clearly the best path is pick the dink then the motor.

You are missing my point. My dink is a small inflatable and would be fine with the xs length. But because I have the longer length I have to lift the motor when I am in waist deep water going to/from a beach. Not terrible but not ideal either having to row into deeper water before starting the motor.

 

[FWT]

You are missing my point. My dink is a small inflatable and would be fine with the xs length. But because I have the longer length I have to lift the motor when I am in waist deep water going to/from a beach. Not terrible but not ideal either having to row into deeper water before starting the motor.

Ah. Thanks for the clarity.

There's a 4 inch difference in length in the two sizes I'm considering. Thankfully it won't make the kind of difference you describe.

 

[backinblue]

Ah. Thanks for the clarity.

There's a 4 inch difference in length in the two sizes I'm considering. Thankfully it won't make the kind of difference you describe.

So if you think 4" doesn't matter why even ask the question? I'm living the exact situation you asked about and I'm telling you it's not ideal. Just buy the long shaft, it's only a few more inches and it will work on any dinghy. Good Luck!

 

[mvweebles]

The question is more about having a shaft longer than spec, not too short.

And FWIW here is the dink that specs an extra short shaft.

https://www.polycraft.com/300-tuffy

Capacity 3 people

This dink specifies a 15" shaft which is standard for regular outboards. I don't see anything about extra-short.

I would definitely go with the manufacturers recommendation.

 

[backinblue]

eProp offers 3 sizes, long, short, and xtra short. I would pick the one that matches my transom.

 

[High Wire]

Torqueedo:

Shaft length: short 62.5 cm / long 75 cm

(24 inches / 29.5)

They measure from the top of the transom to the middle of the gearcase bullet. NO gas OB manufacturer (or dinghy manuf) measures their shaft length that way. Just make sure the prop can't hit the tubes and you will be ok.

 

[BruceK]

So if you think 4" doesn't matter why even ask the question? I'm living the exact situation you asked about and I'm telling you it's not ideal. Just buy the long shaft, it's only a few more inches and it will work on any dinghy. Good Luck!

Ah, the joys of selecting shaft length.

 

[FWT]

They measure from the top of the transom to the middle of the gearcase bullet. NO gas OB manufacturer (or dinghy manuf) measures their shaft length that way. Just make sure the prop can't hit the tubes and you will be ok.

Thank you

My takeaway had become that there seemed to be a measurement method difference between the standard gas outboard measurements and ePropulsion measurements since the lengths quoted here vs ePropulsion don't seem to line up. IE, what does "short" mean in length.

To answer another question, the link I posted was to a manufacturer site, but the recommendation for extra short came from a dealer site, now deleted as that dealer seems to have stopped selling electrics (supply problems?).

 

[mvweebles]

Decent diagram describing how to properly size an outboard. Measurement is from the stop on the mounting bracket (the part that rests atop the transom) to the cavitation plate just above the prop.

Peter

 

[FWT]

Peter: thanks

 

[backinblue]

Rather than trying to understand how outboards are normally measured, if you go to the eProp site, they tell you what length motor you need based on the height of the dinghy transom. They have a diagram showing you exactly what they mean by transom height too. If you assume a "short" eProp is the same as a "short" Merc, you'll be wrong. I would not go by a dinghy manufacturer's recommendation of a "short" shaft length w/o knowing the dimensions of the transom.

 

[FWT]

It seems ePropulsion names their 20 inches shaft "extra small", which is 14 inches using the measurement approach Peter's chart provides, measured to the top of the prop. There is no cavitation plate on this motor. So one might add (or would that be subtract?) something to account for that difference.

The biggest point being something I've never seen reference to anywhere. Don't just take the nominal measurements from ePropulsion lightly when making an order, and get surprised. Dig out the spec measurements.

Which are here.

https://epropulsion.westboundyachts.com/wp-content/uploads/2021/12/Techinical-Drawing_Spirit-1.0-Plus.pdf

 

[FWT]

Rather than trying to understand how outboards are normally measured, if you go to the eProp site, they tell you what length motor you need based on the height of the dinghy transom. They have a diagram showing you exactly what they mean by transom height too. If you assume a "short" eProp is the same as a "short" Merc, you'll be wrong. I would not go by a dinghy manufacturer's recommendation of a "short" shaft length w/o knowing the dimensions of the transom.

We just cross-posted. That's exactly the point I was just making. Agree.

 

[mvweebles]

It seems ePropulsion names their 20 inches shaft "extra small", which is 14 inches using the measurement approach Peter's chart provides, measured to the top of the prop. There is no cavitation plate on this motor. So one might add (or would that be subtract?) something to account for that difference.

The ePropulsion diagram shows 20-inches to the centerline of the prop. Prop is 6-inches in diameter, so it is approximately 17-inches (vs 14-inches) if measured how traditional gas outboards are measured.

Any idea why the ePropulsion does not have a cavitation plate? I too would call the manufacturer to get a better idea of sizing - seems to be something inherently different about the e-motor vs gas.

Peter

 

[backinblue]

The ePropulsion diagram shows 20-inches to the centerline of the prop. Prop is 6-inches in diameter, so it is approximately 17-inches (vs 14-inches) if measured how traditional gas outboards are measured.

Any idea why the ePropulsion does not have a cavitation plate? I too would call the manufacturer to get a better idea of sizing - seems to be something inherently different about the e-motor vs gas.

Peter

Thanks Peter, I agree with you too. Regarding the cavitation plate, the eProp propeller spins at a lower RPM than a gas motor. It's not going to get your dinghy on plane. I'm just guessing, but maybe a cavitation plate, although useful for what it does, may add some drag or other inefficiency and since cavitation is not likely to happen with this motor anyway, why have one?

 

[mvweebles]

Thanks Peter, I agree with you too. Regarding the cavitation plate, the eProp propeller spins at a lower RPM than a gas motor. It's not going to get your dinghy on plane. I'm just guessing, but maybe a cavitation plate, although useful for what it does, may add some drag or other inefficiency and since cavitation is not likely to happen with this motor anyway, why have one?

Running at lower RPM occured to me too If I read correctly, the prop is 6" x 12" so a fairly modest pitch. I don't know how that compares to a 3hp outboard, but seems like it wants some fairly high RPMs. But I just don't know. A call to the manufacturer would likely answer all questions. Clearly the e-outboard is spec'd differently than the gas counterpart. Would be interesting to know why given momentum and traction for e-motors in the future.

Peter

 

[backinblue]

Running at lower RPM occured to me too If I read correctly, the prop is 6" x 12" so a fairly modest pitch. I don't know how that compares to a 3hp outboard, but seems like it wants some fairly high RPMs. But I just don't know. A call to the manufacturer would likely answer all questions. Clearly the e-outboard is spec'd differently than the gas counterpart. Would be interesting to know why given momentum and traction for e-motors in the future.

Peter

It's clearly a larger prop than typical gas, but I'm not an engine engineer so I don't know the reasoning. I assume it's optomized for effciency. Nobody cares about the mpg of a 3 HP gas motor.