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Old 31-03-2020, 14:36   #1
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Diesel to 4 Stroke outboard conversion in displacement hull.

Ahoy there,
I have a Roberts Longboat 21 displacement motor cruiser that currently has a midship mounted 27hp diesel with shaft to the prop achieving 6 knots at 1.5ltrs per hour.
As I am planning a refit of the boat, I am considering removing the diesel and fitting a high thrust 4 stroke outboard in a though hull well with the prop in the same position of the current prop. It would be fixed with the current rudder remaining in use. I also plan to fit a ballast tank and batteries in the keel where the diesel would be. (Hoping for adjustable ballast as the boat is on a trailer).
I am hoping for some input into required hp and any handling concerns for this radical change.
Thanks in advance for any help.
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Old 31-03-2020, 14:53   #2
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Re: Diesel to 4 Stroke outboard conversion in displacement hull.

If it were me I’d go with another diesel; BUT secondly a pair of 9.9 or 15’s 9.9 being preferred. You don’t want to go too big say a single 90 or you’ll be using the engine(s) in a poor power band and ultimately have a shorter service life on it/them. A pair of 9.9’s would be more than enough to get you to hull speed and keep you around 60-75%rpm. Only reason I say a pair is redundancy and acceleration, a single 9.9/15 will get you there also. You’d have to play around with props to really get to that sweet spot and will be dependant on manufacturer, boat hull design, use, preferences etc.

Either way you won’t like the fuel mileage you’re gonna get out of gas outboards vs another onboard diesel. I would really reconsider another diesel.
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Old 31-03-2020, 15:10   #3
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Re: Diesel to 4 Stroke outboard conversion in displacement hull.

Quote:
Originally Posted by Razorcanfly View Post
>>>>>>>>>>
>>>>>>>>>>>>>>>>>

I have a Roberts Longboat 21 displacement motor cruiser that currently has a midship mounted 27hp diesel with shaft to the prop achieving 6 knots at 1.5ltrs per hour.>>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>>>

That sounds incredibly high for fuel consumption.


1.5 litres per hour = 5.68 gallons per hour


I have a 21 hp in our 34 foot boat and get 0.5 gallons per hour


It appears you may have missed a decimal place.


You could compare this lower and, most likely more correct figure, to the fuel consumption of whatever you plan to use.
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Old 31-03-2020, 15:18   #4
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Re: Diesel to 4 Stroke outboard conversion in displacement hull.

Thank you for your response Stu. You seem to have reversed the conversion. 1.5 metric litres converts to just under 0.4 gallons.
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Old 31-03-2020, 15:22   #5
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Re: Diesel to 4 Stroke outboard conversion in displacement hull.

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Originally Posted by Stu Jackson View Post
That sounds incredibly high for fuel consumption.


1.5 litres per hour = 5.68 gallons per hour


I have a 21 hp in our 34 foot boat and get 0.5 gallons per hour


It appears you may have missed a decimal place.


You could compare this lower and, most likely more correct figure, to the fuel consumption of whatever you plan to use.
I think your liters are a lot bigger than my litersmy 3 liter wine bottle is less than 1 gal.
I think 1.5 liters is good number
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Old 31-03-2020, 15:26   #6
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Re: Diesel to 4 Stroke outboard conversion in displacement hull.

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Originally Posted by Razorcanfly View Post
Ahoy there,
I have a Roberts Longboat 21 displacement motor cruiser that currently has a midship mounted 27hp diesel with shaft to the prop achieving 6 knots at 1.5ltrs per hour.
As I am planning a refit of the boat, I am considering removing the diesel and fitting a high thrust 4 stroke outboard in a though hull well with the prop in the same position of the current prop. It would be fixed with the current rudder remaining in use. I also plan to fit a ballast tank and batteries in the keel where the diesel would be. (Hoping for adjustable ballast as the boat is on a trailer).
I am hoping for some input into required hp and any handling concerns for this radical change.
Thanks in advance for any help.
I think 6 knots is pushing the limits for a 21' full displacement hull
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Old 31-03-2020, 15:33   #7
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Re: Diesel to 4 Stroke outboard conversion in displacement hull.

Yes, that is full achievable speed in calm seas and running around 2100rpm.
Most cruising is around 4.5 knots at 1700rpm.
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Old 31-03-2020, 15:55   #8
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Re: Diesel to 4 Stroke outboard conversion in displacement hull.

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I think your liters are a lot bigger than my litersmy 3 liter wine bottle is less than 1 gal.
I think 1.5 liters is good number
3 lt exceeds optimum capacity for a wine bottle.

Reducing the volume enhances the quality of the contents and the cognitive function of the consumer.

700 -760ml is considered target volume.

Multiple containers are possible allowing for flexibility and redundancy of storage and consumption.
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Old 31-03-2020, 16:06   #9
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Re: Diesel to 4 Stroke outboard conversion in displacement hull.

I am trying to consider all pros and cons of the conversion before starting.
The current position of the engine is centred in the main bulkhead and takes considerable room from both the cockpit and cabin, is very noisy and not easily accessed for maintenance.
I expect greater fuel consumption, but hoping to cruise at relatively low revs.
Would fitting dual outboards bring issues with moving away from the keel line and displacement flow lines?
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Old 31-03-2020, 16:18   #10
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Re: Diesel to 4 Stroke outboard conversion in displacement hull.

Quote:
Originally Posted by Razorcanfly View Post
I am trying to consider all pros and cons of the conversion before starting.
The current position of the engine is centred in the main bulkhead and takes considerable room from both the cockpit and cabin, is very noisy and not easily accessed for maintenance.
I expect greater fuel consumption, but hoping to cruise at relatively low revs.
Would fitting dual outboards bring issues with moving away from the keel line and displacement flow lines?
Not really with your setup. Biggest issue will be ensuring they get deep enough into the water. This becomes more an issue with planning hulls. You need to balance the size/power with your cruising speed. Too big an engine and you’ll basically be idling it at hull speed, not good practice for 4s outboards. Just like a diesel you need to get it to proper temp and operating speed. 9.9 high thrust outboards have been pushing plenty of displacement hills around for years.
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Old 31-03-2020, 16:18   #11
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Re: Diesel to 4 Stroke outboard conversion in displacement hull.

Quote:
Originally Posted by Razorcanfly View Post
Ahoy there,
I have a Roberts Longboat 21 displacement motor cruiser that currently has a midship mounted 27hp diesel with shaft to the prop achieving 6 knots at 1.5ltrs per hour.
As I am planning a refit of the boat, I am considering removing the diesel and fitting a high thrust 4 stroke outboard in a though hull well with the prop in the same position of the current prop. It would be fixed with the current rudder remaining in use. I also plan to fit a ballast tank and batteries in the keel where the diesel would be. (Hoping for adjustable ballast as the boat is on a trailer).
I am hoping for some input into required hp and any handling concerns for this radical change.
Thanks in advance for any help.
A well mounted outboard will work

Thx problem with well mounted outboards is exhaust

It fills the boat

You would need a blower or somekinda system to deal with this
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Old 31-03-2020, 16:18   #12
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Re: Diesel to 4 Stroke outboard conversion in displacement hull.

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Originally Posted by Rucksta View Post
3 lt exceeds optimum capacity for a wine bottle.

Reducing the volume enhances the quality of the contents and the cognitive function of the consumer.

700 -760ml is considered target volume.

Multiple containers are possible allowing for flexibility and redundancy of storage and consumption.
I did not say expensive wine I drink a table wine once a night so does wife 3 liter bottles

https://www.totalwine.com/wine/fruit...wPGQ&gclsrc=ds
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Old 31-03-2020, 16:24   #13
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Re: Diesel to 4 Stroke outboard conversion in displacement hull.

Quote:
Originally Posted by Razorcanfly View Post
Ahoy there,
I have a Roberts Longboat 21 displacement motor cruiser that currently has a midship mounted 27hp diesel with shaft to the prop achieving 6 knots at 1.5ltrs per hour.
As I am planning a refit of the boat, I am considering removing the diesel and fitting a high thrust 4 stroke outboard in a though hull well with the prop in the same position of the current prop. It would be fixed with the current rudder remaining in use. I also plan to fit a ballast tank and batteries in the keel where the diesel would be. (Hoping for adjustable ballast as the boat is on a trailer).
I am hoping for some input into required hp and any handling concerns for this radical change.
Thanks in advance for any help.
if it is 4 stroke think a lot on propane conversion. very simple I did it to a 9.9 Johnson. and if a old man like me can, just about anybody can. Look at YT 4sroke conversions, most show lawn mowers but they are basically same motor can be done cheaply
will see some on generators too. have fun stay safe
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Old 31-03-2020, 16:26   #14
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Re: Diesel to 4 Stroke outboard conversion in displacement hull.

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Originally Posted by slug View Post
A well mounted outboard will work

Thx problem with well mounted outboards is exhaust

It fills the boat

You would need a blower or somekinda system to deal with this
My Johnson 9.9 4 stroke exhaust was under water not like 2 strokes.
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Old 31-03-2020, 16:37   #15
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Re: Diesel to 4 Stroke outboard conversion in displacement hull.

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Originally Posted by smbdyiam2 View Post
My Johnson 9.9 4 stroke exhaust was under water not like 2 strokes.
I think he meant an outboard ‘well’ which the OP’s boat does not appear to have. Open transom possibly a fixed swim grid? That would make it easier to mount them with some reinforcement.
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