Al
Guru
- Joined
- Apr 1, 2012
- Messages
- 2,206
- Location
- usa
- Vessel Name
- 'SLO'~BELLE
- Vessel Make
- 1978 Marben-27' Flybridge Trawler(extended to 30 feet) Pilothouse Pocket Cruiser[
Greetings Forum
Curious mind is asking: What is the explanation for the different WOT RPMs for different applications? Recently a shop manual for the Perkins 4-236 engine purchased as a replacement for our Perkins 4-154.
In reading the manual there is a section giving the WOT hp rating. They are as follows
Pleasure-(High Speed) 84 bhp at 2800 RPM (This is the more common settings that I have read and I used to determine the recommended correct wheel measurements)
Pleasure- 76 bhp at 2500 RPM
Commercial- 65 bhp at 2250 RPM
Commercial (Heavy Duty) 54 bhp at 1800 RPM
The reason for the inquiry relates to the subject of “Over Propping”. There are existing forum subjects related to the good and evil of over propping with the majority stating that using the recommended setting of the manufacture is the better choice. As these four are manufactures recommendations it would seem the subject is open for discussion as to what determines each rating. There is none referenced in the manual
Having stated this information an assumption is being made as to the use of our current wheel on the Perkins 4-154 with a 3:1 gear which is 22 X 16. This is very close to the Vicprop formula of 21.5” X 15” for this engine/gear ratio.
The replacement engine the Perkins 4-236 with a 2:1 gear using the Vicprop formula calls for a 20” X 14” wheel. Both of these results are using the Pleasure (high speed) formula. As the boat is far from a high speed hull (6.9 knt hull speed by formula) the thought goes towards the “ Pleasure” of 2500 RPM. with the thinking that the difference in the two wheels recommended may account as compensation for using the current larger (Over Propped) sized wheel which is ou wheel 22” X 16”. in use.
The point of the engine swap is not to go faster but to accomplish the current results at a reduced RPM (Quite) with less fuss. A by product is anticipated in reduced fuel burn. With the current engine we turn 2400 RPM to obtain hull speed.(7 knt +-_ the shaft is turning 800 RPM with a fuel burn of close to 2 gallon per hour . With the replacement engine the same RPM at the shaft is obtained at 1600 RPM. the fuel burn is anticipated at 1 gallon +- per hour based on antidotal information from harbor mates running the same engine/gear ratio at a higher 1800 RPM settings.
A secondary reason is having two wheels on hand over replacing with a new or used smaller wheel.
A third reason is minimal fuel tankage of 74 gallons. Adding additional tankage is out of the question unless above deck mounted tank is considered.
Perhaps this will entice a discussion that will be enlightening to all.
Regards,
Al-Ketchikan (Bridge to Nowhere) Alaska
Curious mind is asking: What is the explanation for the different WOT RPMs for different applications? Recently a shop manual for the Perkins 4-236 engine purchased as a replacement for our Perkins 4-154.
In reading the manual there is a section giving the WOT hp rating. They are as follows
Pleasure-(High Speed) 84 bhp at 2800 RPM (This is the more common settings that I have read and I used to determine the recommended correct wheel measurements)
Pleasure- 76 bhp at 2500 RPM
Commercial- 65 bhp at 2250 RPM
Commercial (Heavy Duty) 54 bhp at 1800 RPM
The reason for the inquiry relates to the subject of “Over Propping”. There are existing forum subjects related to the good and evil of over propping with the majority stating that using the recommended setting of the manufacture is the better choice. As these four are manufactures recommendations it would seem the subject is open for discussion as to what determines each rating. There is none referenced in the manual
Having stated this information an assumption is being made as to the use of our current wheel on the Perkins 4-154 with a 3:1 gear which is 22 X 16. This is very close to the Vicprop formula of 21.5” X 15” for this engine/gear ratio.
The replacement engine the Perkins 4-236 with a 2:1 gear using the Vicprop formula calls for a 20” X 14” wheel. Both of these results are using the Pleasure (high speed) formula. As the boat is far from a high speed hull (6.9 knt hull speed by formula) the thought goes towards the “ Pleasure” of 2500 RPM. with the thinking that the difference in the two wheels recommended may account as compensation for using the current larger (Over Propped) sized wheel which is ou wheel 22” X 16”. in use.
The point of the engine swap is not to go faster but to accomplish the current results at a reduced RPM (Quite) with less fuss. A by product is anticipated in reduced fuel burn. With the current engine we turn 2400 RPM to obtain hull speed.(7 knt +-_ the shaft is turning 800 RPM with a fuel burn of close to 2 gallon per hour . With the replacement engine the same RPM at the shaft is obtained at 1600 RPM. the fuel burn is anticipated at 1 gallon +- per hour based on antidotal information from harbor mates running the same engine/gear ratio at a higher 1800 RPM settings.
A secondary reason is having two wheels on hand over replacing with a new or used smaller wheel.
A third reason is minimal fuel tankage of 74 gallons. Adding additional tankage is out of the question unless above deck mounted tank is considered.
Perhaps this will entice a discussion that will be enlightening to all.
Regards,
Al-Ketchikan (Bridge to Nowhere) Alaska