Derating a 5 hp Johnson to 4 hp. Easy?

D

daijinks

New Member
Joined
10 Aug 2020
Messages
11
Visit site
Hi there,
I own a Johnson 5 hp 2 stroke 1995-6. To qualify for 50% annual discount to use it on British canals with my 4m sib, it has to be 4 hp or below.
Can this be done ? Are the standard 4 hp single or twin like the 5 hp? And carb wise, I know the 5 he model is a derated 8.

Any experience on this old be really welcome.
Cheers
 
daijinks said:
Hi there,
I own a Johnson 5 hp 2 stroke 1995-6. To qualify for 50% annual discount to use it on British canals with my 4m sib, it has to be 4 hp or below.
Can this be done ? Are the standard 4 hp single or twin like the 5 hp? And carb wise, I know the 5 he model is a derated 8.

Any experience on this old be really welcome.
Cheers
Click to expand...
Not sure . As you have mentioned already many outboards shared the same block but just used different carbs to produce different outputs. maybe better to talk to an experienced outboard mechanic for advice.
 
Yes ! What was I thinking about by doing mechanical changes. Doh!
Do I just lie about it.? saying its a 5 hp to insurance as it is but have 4 on the cowl for waterway inspections.
Does the graphics mean anything legally and invalidate insurance? Etc.
 
It's likely that the bhp actually delivered varies from the theoretical bhp in the sales spec anyway. This is common with cars measured on rolling roads. For example new cars aren't bedded in, run-in newish aged cars give good perfomance and as they get older often the power output declines. So a pound to a penny your ouboard isn't giving exactly 5 bhp .it might be 5.5 or 4.5.

I don't know about canal rules but I'd be suprised if they made a rule that would require actual engineering measurement of the power output of an outboard because that would be prohibitively expensive to enforece. Is it possible that the rule refers to the manufacturer's rated output? In which case altering the actual output might not help.

I guess that the evil they are trying to avoid is guys with 150 HP outboards on their speedboats trying them out on the canal. Painting over the cowl in a tasteful shade to match your boat might be enough to prevent the boys in blue (or whatever colour the canal spped cops wear) from ever taking the slightest notice of you. As long as you don't go so fast as to attract their attention.
 
daijinks said:
Hi there,
I own a Johnson 5 hp 2 stroke 1995-6. To qualify for 50% annual discount to use it on British canals with my 4m sib, it has to be 4 hp or below.
Can this be done ? Are the standard 4 hp single or twin like the 5 hp? And carb wise, I know the 5 he model is a derated 8.
.
Any experience on this old be really welcome.
Cheers
Click to expand...

The 4 hp engines are twin cylinder engines but a smaller cubic capacity. 87cc compared with 164 cc for 5, 6 and 8 hp.

I am sure it would be possible to fit a restrictor between carb and the manifold which would reduce the power. Or perhaps a limiter on the throttle cable or linkage movement but the difficulty would be determining the size of the restrictor or the extent to which the throttle movement is limited.

Then you would have to be able to convince the CRT that you do genuinely have a 4 hp engine.
Perhaps you should fit the cowl from a 4 hp deluxe .. I think one will fit.
 
The HP sa specified is the maximum HP that the engine can develop at a specified engine speed.

The HP that would be develop also depends on the resistance.

You can limit the engine speed thus the HP developed by over propping the engine.

The other issue you will have is that the authority will look at the engine and see a 5hp badge and even through you nay have derated the engine you would have to convince the authorities that it is a 4 hp and not a 5 hp as on the label.

As Paul says get some stickers and problem solved as far as the authorities are concerned.

You may also have to change the specification.

I have a 16 hp outboard for my dingy but the maximum hp I can use without special licensing is 15 hp, so I had a similar issue.
 
Rogershaw said:
The HP sa specified is the maximum HP that the engine can develop at a specified engine speed.

The HP that would be develop also depends on the resistance.

You can limit the engine speed thus the HP developed by over propping the engine.

The other issue you will have is that the authority will look at the engine and see a 5hp badge and even through you nay have derated the engine you would have to convince the authorities that it is a 4 hp and not a 5 hp as on the label.

As Paul says get some stickers and problem solved as far as the authorities are concerned.

You may also have to change the specification.

I have a 16 hp outboard for my dingy but the maximum hp I can use without special licensing is 15 hp, so I had a similar issue.
Click to expand...
Yes I get it now, thanks all great and sound advice here.
I guess, the way the font style of 5 is presented to look like a 4 can fool the authority (as there's no law how a make and model and it's bhp text, is displayed as long as it's clearish, could look totally crap but so what?). And surely there nothing stopping one writing the number 4 all over the place.
My second engine (for the sea) is a mariner 15 2 stroke, oldish 1981. Only just bought it but yet to collect ( lockdown laws).
I'm looking for the best manual for these, I read OEM is the one to get but where from? Nothing's on eBay etc. ( perhaps online somewhere?)
Also a set of used remote gear and throttle cables with couplings. 3m long.
I'm quite new to all this and this is my way out of urban lockdown rut to freedom and fun next year.
Any pointers most welcome.
 
daijinks said:
Yes I get it now, thanks all great and sound advice here.
I guess, the way the font style of 5 is presented to look like a 4 can fool the authority (as there's no law how a make and model and it's bhp text, is displayed as long as it's clearish, could look totally crap but so what?). And surely there nothing stopping one writing the number 4 all over the place.
My second engine (for the sea) is a mariner 15 2 stroke, oldish 1981. Only just bought it but yet to collect ( lockdown laws).
I'm looking for the best manual for these, I read OEM is the one to get but where from? Nothing's on eBay etc. ( perhaps online somewhere?)
Also a set of used remote gear and throttle cables with couplings. 3m long.
I'm quite new to all this and this is my way out of urban lockdown rut to freedom and fun next year.
Any pointers most welcome.
Click to expand...

About the only "official" source for Johnson and Evinrude OEM manuals , apart from very recent models, is https://www.outboardbooks.com/

Your Mercury/Mariner might be able to help with a manual for the Mariner but at I reckon you'll be very lucky to get one for an engine that old
 
IMHO
1) If you want to use a 5hp motor then , like every one else, pay the fee
2) If that would ruin your enjoyment, then sell the 5hp and get a 4hp motor
3) If that is not possible think about changing hobbies.
So this forum is now about how to circumvent rules and regulations that cost us money. What next a thread on the best way to falsify the costly insurance documents my marina insists I produce each year or does anyone know how to hotwire the electric points so I don't have to pay for the electric I use?
Sorry if this offends anyone. just my opinion.
 
The only catch is the factory i.d. plate, which will say it is 5hp, if they bother to look.

barclay. He is actually asking to see of he can reduce the hp? Faking it came from the answers...

Edit: I actually have a 4hp Twin Evenrude in the shed, bought new in the 60s. Similar engine IIRR, Not used it for 15yrs and I doubt it will run, but pm me if it can help.
 
Last edited:
barclay said:
IMHO
1) If you want to use a 5hp motor then , like every one else, pay the fee
2) If that would ruin your enjoyment, then sell the 5hp and get a 4hp motor
3) If that is not possible think about changing hobbies.
So this forum is now about how to circumvent rules and regulations that cost us money. What next a thread on the best way to falsify the costly insurance documents my marina insists I produce each year or does anyone know how to hotwire the electric points so I don't have to pay for the electric I use?
Sorry if this offends anyone. just my opinion.
Click to expand...
Good grief it's not like the forum is advising him on a bank job. A little bending of some rules. I'm with the paint the cowl and plaster some stickers on it. Nobody will, or could, test the power output.
 
daijinks said:
I own a Johnson 5 hp 2 stroke 1995-6. To qualify for 50% annual discount to use it on British canals with my 4m sib, it has to be 4 hp or below.
Click to expand...
It that a Canal and River Trust discount or is it an insurers discount?
 
moondancer2 said:
Good grief it's not like the forum is advising him on a bank job. A little bending of some rules. I'm with the paint the cowl and plaster some stickers on it. Nobody will, or could, test the power output.
Click to expand...
It would be telling an untruth. Some people do not like doing that.

FWIW the 4 hp power head is different to the 5,6,8 power head. You not need to test it to tell the difference. It would be pretty obvious that the engine had a 5 hp ( or larger ) powerhead rather than a 4
 
VicS said:
It would be telling an untruth. Some people do not like doing that.

FWIW the 4 hp power head is different to the 5,6,8 power head. You not need to test it to tell the difference. It would be pretty obvious that the engine had a 5 hp ( or larger ) powerhead rather than a 4
Click to expand...
It seems ok for our leaders. Especially in a limited way.
 
You must log in or register to reply here.

Other threads that may be of interest

T
Notices to Mariners - Week 50
Replies
2
Views
744
tillergirl
T

Share this page

Top