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Tackrun · 07-10-2020, 15:52

Hi all, recently bought a 1977 Pearson 30 with a locked up atomic 4 that I've decided to scrap and replace with an electric motor. Unfortunately I'm not having much luck finding a particular spec:

Does anyone know what the optimal RPM that the atomic 4 is designed to run at in order at to achieve hull speed?

Thanks!!

Joefiss · 07-10-2020, 16:29

Quote:

Originally Posted by Tackrun

Hi all, recently bought a 1977 Pearson 30 with a locked up atomic 4 that I've decided to scrap and replace with an electric motor. Unfortunately I'm not having much luck finding a particular spec:

Does anyone know what the optimal RPM that the atomic 4 is designed to run at in order at to achieve hull speed?

Thanks!!

This might be of interest. https://www.moyermarineforum.com/for...el+consumption

Svsumurun · 07-10-2020, 16:49

Depends on the prop .... we had a P30 and it really didn't need much throttle to hit hull speed ( which, btw the boat could go WAY faster than that) .. 1500 rpm on ours would cruise at hull speed all day long..

Tackrun · 07-10-2020, 16:52

Awesome thank you!! Was yours a shaft drive with 2 blade prop?

Svsumurun · 07-10-2020, 17:16

Yes it was

Tackrun · 07-10-2020, 17:32

One last thing if you don't mind - do you remember about what that max speed & rpm you could run was?

Scubaseas · 07-10-2020, 18:45

You can not get a displacement hull to go faster than the hull speed which is determined by water line length. In the 77 pearson 30 it's 25'LWL and a hull speed of 6.7kts. You could pump 100HP through a prop and not go faster than 6.7kts. The more power you supply the more the stern will squat into the water.

RPM and speed will depend on the size and pitch of your prop.

https://www.westerbeke.com/advanced%...literature.pdf

**Jim Cate** · 07-10-2020, 18:56

IIRC most Anemic Four trannies were direct drive, ie no reduction in rpm from motor. As a result, most yachts with A-4s had rather small diameter props. If you are changing to an electric drive, it would behoove you to put a considerably larger diameter prop in service, for it will be far better in converting electrical energy into thrust.

Jim

JimsCAL · 07-10-2020, 18:56

Most A4s were direct drive. That means the prop turns much faster than ideal for best efficiency. A new power plant, electric or diesel, would allow for a slower turning, larger, and more efficient prop. You are really looking at clean sheet of paper. Don't focus too much on the A4 installation.

**Jim Cate** · 07-10-2020, 20:09

See above two posts for an example of synchronicity.

Jim

**GordMay** · 08-10-2020, 08:30

From the
Atomic 4 OPERATION AND MAINTENANCE MANUAL
➥ https://www.westerbeke.com/operator%...erator_man.pdf

Maximum RPM = 3500
Reduction Gear Ratio(s): 2:1(UJR), or 1:1, 1.29:1, 1.67:1, 2:1 (UJR VD)

guyrj33 · 08-10-2020, 08:48

Quote:

Originally Posted by Scubaseas

You can not get a displacement hull to go faster than the hull speed which is determined by water line length. In the 77 pearson 30 it's 25'LWL and a hull speed of 6.7kts. You could pump 100HP through a prop and not go faster than 6.7kts. The more power you supply the more the stern will squat into the water.

RPM and speed will depend on the size and pitch of your prop.

https://www.westerbeke.com/advanced%...literature.pdf

perhaps you can explain why i've been able to exceed hull speed in my Pearson 10M? Even hitting 12 Knots one day. 29' waterline.

Scubaseas · 08-10-2020, 09:50

Quote:

Originally Posted by guyrj33

perhaps you can explain why i've been able to exceed hull speed in my Pearson 10M? Even hitting 12 Knots one day. 29' waterline.

Speed over ground or speed through water? 12 knots on a 28.3' water line is a remarkable statement. If you go faster than the wave you are making you are planing which is hard to do in a Pearson 30. Or a 10M for that matter. Getting a 10M to plane might be possible but I doubt you can get a P30 to do it.

I've hit 11 knots in my Pearson 323 but I had to be in a 5kt fair current to do it.

Look up "hull speed" on Wiki for a brief discussion.

If you have a boat with a 30hp motor and try upping it to 50hp it won't make the boat go any faster but it will push the hull down more into the water.

guyrj33 · 08-10-2020, 10:04

Quote:

Originally Posted by Scubaseas

Speed over ground or speed through water? 12 knots on a 28.3' water line is a remarkable statement. If you go faster than the wave you are making you are planing which is hard to do in a Pearson 30. Or a 10M for that matter. Getting a 10M to plane might be possible but I doubt you can get a P30 to do it.

I've hit 11 knots in my Pearson 323 but I had to be in a 5kt fair current to do it.

Look up "hull speed" on Wiki for a brief discussion.

If you have a boat with a 30hp motor and try upping it to 50hp it won't make the boat go any faster but it will push the hull down more into the water.

I’m referring to speed through the water, speed over ground has nothing to with hull speed discussions.
12 kt in a 10m is very exciting.

thomm225 · 08-10-2020, 10:44

Quote:

Originally Posted by Tackrun

Hi all, recently bought a 1977 Pearson 30 with a locked up atomic 4 that I've decided to scrap and replace with an electric motor. Unfortunately I'm not having much luck finding a particular spec:

Does anyone know what the optimal RPM that the atomic 4 is designed to run at in order at to achieve hull speed?

Thanks!!

Call Moyers Marine.

They know all about the Atomic 4 Engine

https://moyermarine.com/