Page 1 of 2

ST21 question for Jacques

Posted: Wed Sep 11, 2019 5:13 am
by glossieblack
Jacques, for ST21 with a 1.5:1 reduction trannie and a 10" diameter prop, what pitch do you recommend? Thanks, M.

Re: ST21 question for Jacques

Posted: Wed Sep 11, 2019 6:58 am
by cape man
What are the max rpms on the diesel engine and the e drive?

Re: ST21 question for Jacques

Posted: Wed Sep 11, 2019 10:16 am
by jacquesmm
I deleted my reply: I did not realize the question came form Glossieblack.

This is for an electric motor but that does not matter. What matters is rpm, diameter and pitch.
I did my own calculations and arrived at the same results than what Atkins specifies for similar boats.
Give me a minute to go through my notes.

Here it is:
I would like a larger prop but we are limited by the depth of the box keel. We are stuck with 10" diameter and will compensate by having sufficient pitch and turning it fast. Atkins made the same choices for similar boats. I reach the same conclusion than he does: a 10" diameter by 10" pitch turning between 2600 and 3000 rpm. Atkins goes for 12" pitch because he used old slower turning engines but if I want to get 15+ HP out of the Kubota diesel I specify, we must run it faster therefore less pitch.
Since you have an electric drive, you can get more torque at lower rpm and could go for 10x12 pitch at 2000 rpm.
I don't think you can find a larger pitch prop in that diameter, I would see what is available question of pitch in 10" diameter and take it from there.

Re: ST21 question for Jacques

Posted: Wed Sep 11, 2019 10:46 am
by jacquesmm
glossieblack wrote:
Wed Sep 11, 2019 5:13 am
Jacques, for ST21 with a 1.5:1 reduction trannie and a 10" diameter prop, what pitch do you recommend? Thanks, M.

Probably more than 12" pitch but that kind of prop does not exist that is why I specify a 1:1 gear.
If you find a 10" prop with more pitch, you can use that 1.5:1 but I did some research years ago and could not find the proper prop.
10x10 is what I could find, that is why I specify 1:1 gear.
BTW, what is your recommended cruise rpm?
I thought you used an electric drive.

Re: ST21 question for Jacques

Posted: Wed Sep 11, 2019 8:45 pm
by glossieblack
Jacques, ST21 Skinnydip has been built with a parallel hybrid drive train based on a 20hp Kubota diesel and a 3-4hp electric motor in parallel.

The diesel drive subsystem will be used most of the time, and will be the only one capable of producing top boat speed. The 10" diameter propeller should be pitched for the diesel drive.

The e-drive will be used in areas restricted to electric motors and with a speed limit of knots. More that 1000 rpm at the prop is unlikely to required.

You may recall that I was unable to find a 1:1 mechanical transmission for the 20 hp Kubota anywhere, so I opted for a 1:1 hydraulic one.

I've been sea trialling a 20hp diesel Kubota with a Hurth ZF 45C 1:i transmission without success. Progressively stronger damper plates have each failed in less than one hour. It seems that a large hydraulic box like the ZF 45C, rated to 300hp, is incompatible with a 20hp diesel engine.

So I've decided to sell the 1:1 hydraulic box and replace it with a Twin Disc 1.5:1 mechanical one. This is a combination used by Nanni and Beta.

The 20hp Kubota can be continuously used up to 3,000 rpm, so with a 1.5:1 box, a 10" diameter prop can be turned up to 2000 rpm.

I realise the pitch of a 10" prop on ST21 is necessarily experimental.

The question I'm asking is: for ST21, what is the likely optimal pitch for a 10" prop spinning at 2000 rpm?

Thanks.

Re: ST21 question for Jacques

Posted: Thu Sep 12, 2019 9:32 am
by jacquesmm
Thanks, that is clear.
i was following the damper plate tribulations for a while but was confused by the talks of electric and mechanical drive at the same time.
At 2000 rpm, a 10" prop should have a 12" pitch.
BTW, this is what Atkins specifies for similar hulls.

Here is a link to a variation on the ST21 fitted with a 25 HP Kubota, 1.5:1 gear. Exactly the same hull but much heavier.
http://stormportboatworks.com/oystercrackerspecs.html
It is one of our XF20 builders and he mentions our plans.
He has a 10" prop but could only find a 10" pitch cupped. I was on the boat and she reached 15 knots top speed (18 mph). She would go faster with a custom 12" pitch. She is much heavier than the ST21.

There are pictures on that site.

Re: ST21 question for Jacques

Posted: Fri Sep 13, 2019 4:47 am
by glossieblack
Thanks Jacques for the really helpful answer.

I've tried a number of times to contact Stormport Boat Works to find out what ratio trannie was used when achieving 15 knots, but to no avail.

I'm pressing ahead with the 1.5: trannie, and can get a 10" diameter RH prop built locally with either a 12" or 13" pitch.

What's your hunch - 12 or 13 pitch?

Re: ST21 question for Jacques

Posted: Fri Sep 13, 2019 8:56 am
by jacquesmm
I would stay with 12", that is what my calculations show.

Re: ST21 question for Jacques

Posted: Fri Sep 27, 2019 2:19 am
by glossieblack
For future reference, if anybody wants to build ST21 with a 1.5:1 transmission and a 10" diameter x 12" pitch propeller, RL Marine located in Brisbane, Australia can make one for you.

I received mine in the post today. :D

Image

Re: ST21 question for Jacques

Posted: Fri Sep 27, 2019 2:27 am
by OrangeQuest
Very nice looking prop!