• Untitled Document

    Join us on March 29rd, 7pm EST

    for the CBEC Virtual Meeting

    All EYO members and followers are welcome to join the fun and get to know the guest speaker!

    See the link below for login credentials and join us!

    March Meeting Info

    (dismiss this notice by hitting 'X', upper right)

What should the max cruise speed under power be?

Ace

Junior Member
What should the max cruise speed under power be for an 1972 e32' with fully functional gas motor.

We bought a bought our boat with a very poorly functioning Palmer P60 after years of failed attempts we could never return it to a state of dependability. We elected to convert the boat to electric power which has been broadly successful.

Only now that things are up and running has it occurred to me that I don't know what original speed numbers are so that I can measure the success of our project. When I say max cruise speed I mean the greatest functional speed A reasonable person would expect to run the boat under engine power for a sustained period of time. Also that what RPMs that would be accomplished would be a great number if available.

thank you in advance
 

Christian Williams

E381 - Los Angeles
Senior Moderator
Blogs Author
Five to six knots.

An M25 diesel makes a lot of noise on an E32-3 if pushed hard, and although the boat could hit 6.5 or so in flat water, I found the best balance for long distances ("the greatest functional speed A reasonable person would expect to run the boat under engine power for a sustained period of time") to be 5-6 with a two-blade prop.

Loren's boat (next post) is slippery. My E38 barely touches 7 knots with its three-blade at Wide Open Throttle on the M40. I typically run at 6.5 knots in a normal seaway.

I wonder how other 38s do.
 
Last edited:

Loren Beach

O34 - Portland, OR
Senior Moderator
Blogs Author
Theory and Reality

There are several calculators on line or you can run the formula yourself.
Also a nice explanation at Wikipedia.

I just found and tried this one.
http://www.samoht.com/spa/boatperfcalc.htm

Sure enough, it came up with 7.1. That's what we can motor at continuously (with a very clean bottom).

Give it a try and see what your boat speed should theoretically be.

Without looking up the data, my SWAG would be about 6.2 or more kts for an E-32-2.

Loren
 
Last edited:

Ace

Junior Member
Thanks

Thanks guys.i'm going to post a few pictures of the project now near substantial completion.
 

paul culver

Member III
Thanks guys.i'm going to post a few pictures of the project now near substantial completion.

Always looking forward to feedback from converters to electric propulsion. I think the rule of thumb for displacing boats is max hull speed equals 1.4 times square root of length on the waterline. I'm sure google will correct me if I'm wrong.

Paul
E29 "Bear"
 

u079721

Contributing Partner
Five to six knots.

An M25 diesel makes a lot of noise on an E32-3 if pushed hard, and although the boat could hit 6.5 or so in flat water, I found the best balance for long distances ("the greatest functional speed A reasonable person would expect to run the boat under engine power for a sustained period of time") to be 5-6 with a two-blade prop.

Loren's boat (next post) is slippery. My E38 barely touches 7 knots with its three-blade at Wide Open Throttle on the M40. I typically run at 6.5 knots in a normal seaway.

I wonder how other 38s do.

On our 1989 E38 we had a three blade Max prop. Wide open was 2800-2900 rpm, which gave about 7.3 kts, and we cruised at around 2450 or so, which IIRC gave me something like 6.8 kts. On one delivery up north we motored or motor sailed 152 nm in 21.5 hours, which averages out to just over 7.0 kts
 

Mark F

Contributing Partner
Blogs Author
Hi Ace,

You will need to know your motor output, reduction ratio, prop size and pitch. Did you buy a motor/controller unit from a manufacture? If so they will be able to give you solid numbers.

In general for longer motor trips you will what to keep your "cruising speed" lower than a typical gas motor. On my E27 with a 5kw motor, 2.5 to 1 reduction and a 13x13 fixed three blade prop I aim for 4kts cruising speed. At 4kts boat speed in smooth water I use 20 amps.

My guess is you will want to change your prop.

What battery are you using?
 
Last edited:

907Juice

Continuously learning
after reading some comments, I think I've been doing it wrong. I'm not so concerned by the speed cause I have a fairly small boat. On a decent clip with my green skills is about 5. My piggy back question should be at what rpm should my engine run. My stamp reads 3400 continuous speed. The internet seems to be split on cruising speed being lower and continuous speed being, well... continuous.

Hope me nobody minds me jumping in.

attachment.php
 

Attachments

  • IMG_2232.jpg
    IMG_2232.jpg
    86.6 KB · Views: 417
Last edited:

Alan Gomes

Sustaining Partner
after reading some comments, I think I've been doing it wrong. I'm not so concerned by the speed cause I have a fairly small boat. On a decent clip with my green skills is about 5. My piggy back question should be at what rpm should my engine run. My stamp reads 3400 continuous speed. The internet seems to be split on cruising speed being lower and continuous speed being, well... continuous.

Hope me nobody minds me jumping in.

attachment.php
I have the identical engine. I cruise it at about 2800-2900 rpm. Max rpm for that engine is 3600, so my cruising rpm is about 80% of that. You should occasionally wind it up higher for a good, hard run to blow out the carbon.

Here is what the Yanmar manual states:

"The GM series engines are designed to be operated at a maximum throttle (3600 rpm) for less than 5% of total engine time (30 minutes out of every 10 hours) and cruising speed (3400 rpm or less) for 90% of total engine time (9 out of every 10 hours)."
 

u079721

Contributing Partner
after reading some comments, I think I've been doing it wrong. I'm not so concerned by the speed cause I have a fairly small boat. On a decent clip with my green skills is about 5. My piggy back question should be at what rpm should my engine run. My stamp reads 3400 continuous speed. The internet seems to be split on cruising speed being lower and continuous speed being, well... continuous.

Great question. Most places I have read suggest that cruising rpm ought to be about 80% or so of the suggested max rpm. BUT, it's important to find out if your engine can reach the designed max rpm before using this value. If anyone has changed your prop, you may now be under or over propped. If you can't get up to the suggested max rpm, then your prop is too big (in diameter, or pitch, or number of blades), and if you reach it too easily, with too low an ultimate speed, you may be under propped. If you are over propped then just using 80% of what the book suggests as max rpm will be overloading the engine if you try to cruise at that level.
 

Ace

Junior Member
Sounds like I am not quite there

Thanks. We are getting 5 knots max in no wind and flat seas. Next step is to add pitch to the prop then posssibly change the reducer from 1:3 to 1:2.5.

Not helping with this this is that presently we need to plug into the motor controller to see the rpm on the motor then divide by 3 in my head.
 

Alan Gomes

Sustaining Partner
Great question. Most places I have read suggest that cruising rpm ought to be about 80% or so of the suggested max rpm. BUT, it's important to find out if your engine can reach the designed max rpm before using this value. If anyone has changed your prop, you may now be under or over propped. If you can't get up to the suggested max rpm, then your prop is too big (in diameter, or pitch, or number of blades), and if you reach it too easily, with too low an ultimate speed, you may be under propped. If you are over propped then just using 80% of what the book suggests as max rpm will be overloading the engine if you try to cruise at that level.
Good points, Steve. I'd also add that the tach used on the Yanmar 1GM, or on mine at least, is not very accurate. At some points in its range mine is off by as much as 250 rpm. I created a tach "deviation table" with the aid of an inexpensive laser tach: http://www.harborfreight.com/digital-photo-sensor-tachometer-66632.html
 
Top