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Old 01-05-2016, 04:05   #16
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Re: Engine Size Considerstion

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Originally Posted by valhalla360 View Post
The issue isn't so much acceleration as the engine bogging down as you punch thru a wave and then it can't recover before the next wave. A bigger engine can muscle thru that without bogging down.
I think that's what the acceleration is. When you are hit by a wave, you can get back up to speed before the next wave. The same term is used when comparing the performance difference between 2 & 3 blade props.

I hope RC was not thinking that the better acceleration was for drag racing in the anchorage
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Old 01-05-2016, 04:45   #17
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Re: Engine Size Considerstion

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Originally Posted by hoppy View Post
I think that's what the acceleration is. When you are hit by a wave, you can get back up to speed before the next wave. The same term is used when comparing the performance difference between 2 & 3 blade props.

I hope RC was not thinking that the better acceleration was for drag racing in the anchorage
Agreed but then we should clarify it's the speed of the prop (RPM at the prop) more so than the boat. When you hit a wave, the boat slows down which means the prop loads up more as it's forced to slip.
- If you are geared like a tractor, the prop loads up but the engine/transmission/prop combination has enough torque that it keeps spinning at close to the same RPM (there will always be a little loss), which ideally should be near peak torque/HP for the engine. Because the RPM loss was small and in the ideal RPM range, it quickly recovers to the desired RPM.
- If you are geared like a Ferrari, the prop loads up and the engine RPM drops noticeably. If you are under-sizing the engine, it can drop the RPM to where you have less torque and the engine struggles to get back up to RPM before the next wave. This can lead to a situation where you can't get the engine into an RPM that ideal range where the engine can put out the rated HP/torque.

(To repeat, you can't take it too extreme or during calm conditions, the engine will be screaming at high RPM just to make normal cruising speed. Going from 38 to 35hp I wouldn't expect this to be an issue.)
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Old 01-05-2016, 05:00   #18
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Re: Engine Size Considerstion

Unless your boat is much heavier than most boats your size I would not go with the bigger engine. Check the gear ratio of the current gear box and discuss with Beta and a prop guy whether that should be duplicated. If you are generally happy with the performance of your boat the larger engine will have minimal advantages. I agree with the poster that noted that a new engine will likely have more "healthy horses" than the old one.
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Old 01-05-2016, 05:11   #19
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Re: Engine Size Considerstion

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Originally Posted by Rustic Charm View Post
Ok, will do, and to avoid Hurth gearboxes.
I've got a hurth HBW100, touch wood its ok... I think a lot of the issues with them are that they choose to spec them for pleasure duty rather than Comercial, Then drive them far to hard in a hot enginebox with no airflow. Essentially they are air cooled, unless you fit an aftermarket watercooler to them.

Mine is well inside the Comercial rating, maybe thats why it has survived so long?
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Old 01-05-2016, 05:51   #20
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Re: Engine Size Considerstion

Hello Rustic Charm,
Been through the process of changing motors over on a couple of our boats now. There are a few things you need to mindful of when considering what motor to put in other than what had already been raised. Idealy you swap out for the identical size and footprint. If not here are a few things that we were not aware of till after we had pulled out our first motor.

1) We upgraded our old 27 HP Yanma (which was no longer in production) and went to a 40HP. The old motor while adequate was to small for the boat. Rough rule of thumb allow 1HP for every foot of boat.

2) Ensure that you motor is matched to the gearbox.

3) While your old prop may match your current prop you will need to ensure it matches whatever you end up. Any of the major aussie prop manufactuers will only be happy to assist from my experience.

4) We found that as we increased the size of the motor the footprint of the motor was larger and hence we had to increase the engine bed mounts which required a shipwright to do the job to a saftisfactory level.

5) The water inlet also had to be inreased as well as the exhaust size when then entailed making a larger hole at the exhaust hole in the hull.

6) The exhaust outlet on the new motor was in a different location to the old motor and we had to have a pipe fabricated to fit in the new position.

7) New motors normally come with new electric starter panel and wiring harness. Ensure that your old electrics are compatable with the new motor. A lot of our electrical items had been sleeved into the old motor wiring harness IE exhaust fans and these were disconnected by the mechanic and not reconnected when re-installed.

8) When we finally took the old motors out we took the time to remove old redundant wiring etc then cleaned and painted the engine bay. When the motor is out its a great time to do any modifications required.

9) On our last adventure swapping motors out the actual cost of having a proffesional mechanic was very affordable and time efficient.

10) Nearly forgot something important. Acess to remove and install a new motor and gearbox. We were very fortunate that this was pretty straight forward in our circumstance but I have seen first hand cockpits having to be cut open to remove and replace the motor and then repaired when the job is completed.

11) Check the size of the alternator coming with the new motor. Some come with one that is just sufficient for the job. We found as a rule of thumb that a 100 amp model will suffice in most instances.

12) For future ease of maintenance check ease access for changing water impellor access. Some motor models are just ludicrous. Also check ease of replacing alternators, water pumps etc.

Hope this is of some interest and assistance.


Greg and Sue
Sunshine
Lagoon 410S2
Currently Hervy Bay Queensland Aus.
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Old 01-05-2016, 05:53   #21
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Re: Engine Size Considerstion

It would appear that my engine is not a 38hp but a 43hp. Not that there's that much difference, but probably make sense for me to get either a 38 or a 45 in kubota.
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Old 01-05-2016, 06:02   #22
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Re: Engine Size Considerstion

There are quite a few calculators on the web to help you determine the optimum horsepower based on your boat's water line, displacement, transmission ratio and prop. There's a great one at www.boatdiesel.com.

I would suggest, however, that when running these calculators, that you assume about a 3-5 horsepower loss for accessories. Most manufacturers test their engines without alternators, water pumps, etc and regardless of what they say the output is, you'll almost certainly get something less. This can be important when you're trying to determine how much horsepower you need.

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Old 01-05-2016, 07:39   #23
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Re: Engine Size Considerstion

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Originally Posted by Rustic Charm View Post
-----
Tentatively comparing the engine bay sizes today I think the 50 will fit.
-----
If the fit is going to be tight don't forget to allow yourself enough room to work on the engine, changing impellers, filters, belts, etc.
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Old 01-05-2016, 10:06   #24
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Re: Engine Size Considerstion

Quote:
Originally Posted by Rustic Charm View Post
It would appear that my engine is not a 38hp but a 43hp. Not that there's that much difference, but probably make sense for me to get either a 38 or a 45 in kubota.
Better to get close to the original motor's hp. Being steel, your boat needs more hp than a similar sized glass boat.
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Old 02-05-2016, 05:53   #25
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Re: Engine Size Considerstion

Quote:
Originally Posted by ozsailer View Post
Hello Rustic Charm,
Been through the process of changing motors over on a couple of our boats now. There are a few things you need to mindful of when considering what motor to put in other than what had already been raised. Idealy you swap out for the identical size and footprint. If not here are a few things that we were not aware of till after we had pulled out our first motor.

1) We upgraded our old 27 HP Yanma (which was no longer in production) and went to a 40HP. The old motor while adequate was to small for the boat. Rough rule of thumb allow 1HP for every foot of boat.

2) Ensure that you motor is matched to the gearbox.

3) While your old prop may match your current prop you will need to ensure it matches whatever you end up. Any of the major aussie prop manufactuers will only be happy to assist from my experience.

4) We found that as we increased the size of the motor the footprint of the motor was larger and hence we had to increase the engine bed mounts which required a shipwright to do the job to a saftisfactory level.

5) The water inlet also had to be inreased as well as the exhaust size when then entailed making a larger hole at the exhaust hole in the hull.

6) The exhaust outlet on the new motor was in a different location to the old motor and we had to have a pipe fabricated to fit in the new position.

7) New motors normally come with new electric starter panel and wiring harness. Ensure that your old electrics are compatable with the new motor. A lot of our electrical items had been sleeved into the old motor wiring harness IE exhaust fans and these were disconnected by the mechanic and not reconnected when re-installed.

8) When we finally took the old motors out we took the time to remove old redundant wiring etc then cleaned and painted the engine bay. When the motor is out its a great time to do any modifications required.

9) On our last adventure swapping motors out the actual cost of having a proffesional mechanic was very affordable and time efficient.

10) Nearly forgot something important. Acess to remove and install a new motor and gearbox. We were very fortunate that this was pretty straight forward in our circumstance but I have seen first hand cockpits having to be cut open to remove and replace the motor and then repaired when the job is completed.

11) Check the size of the alternator coming with the new motor. Some come with one that is just sufficient for the job. We found as a rule of thumb that a 100 amp model will suffice in most instances.

12) For future ease of maintenance check ease access for changing water impellor access. Some motor models are just ludicrous. Also check ease of replacing alternators, water pumps etc.

Hope this is of some interest and assistance.


Greg and Sue
Sunshine
Lagoon 410S2
Currently Hervy Bay Queensland Aus.
Thank you for your considered response. Lots there for me.

Just on the 'access'. Last year I cut a square hole in the cockpit to put my motor back in with. I Sika and bolted a temp floor down on to it. But I also made up a new hatch and cover which will go down for removing and installing a new motor. It will also give me a hatch in future to work on the engine at any time, including at sea.
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Old 02-05-2016, 05:56   #26
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Re: Engine Size Considerstion

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Originally Posted by Terra Nova View Post
Better to get close to the original motor's hp. Being steel, your boat needs more hp than a similar sized glass boat.
My preferred supplier only does a 35hp or 50hp. Given my VP was 43 do you think the 50 would be too much?

To get a 38 , 45, I'd have to go with a Beta
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Old 02-05-2016, 05:58   #27
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Re: Engine Size Considerstion

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If the fit is going to be tight don't forget to allow yourself enough room to work on the engine, changing impellers, filters, belts, etc.
The width of all the considered kubotto's up to and including a 50 are all similar width to my 43hp VP. The 45 and 50 however are longer.
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Old 02-05-2016, 06:06   #28
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Re: Engine Size Considerstion

How did she go with the old 43? Did you ever feel like you needed more power? Or did you feel like you couldnt usefully use full revs on the old VP. Ie punching into strong winds you needed to slow the engine down, or at least not use near 80% rpm because the boat was thumping so bad?

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Old 02-05-2016, 06:45   #29
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Re: Engine Size Considerstion

I could not find any information on your boat... A Bieroc 36?

What is the displacement? What is the LWL? What sort of bottom shape does it have? It is a ketch, and from your avatar it appears to have a fair amount of windage.

These are the kinds of questions one must ask before being able to give a good recommendation on what size engine you need.
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