4hp Johnson remote fuel tank and prop change

colinc

Recruit
Joined
Jan 9, 2007
Messages
3
Hi.

I have a 4hp Johnson engine (BJ4BRCCS - serial# 7641969) which I believe is a 1988 model. I'd very much like to increase the prop pitch or diameter to get a little more grunt and a little less thrashing. I'd also quite like to use a remote fuel tank so that I'm not having to stop to top up every 3/4 of an hour or so.

Are either of these things particularly difficult to achieve? I'm not at all sure whether my engine has a mechanical fuel pump or not and a web search for "johnson 4hp propellors" just leaves me more confused than ever.

I'm in the UK, if anyone's in a position to post a source for a new prop and/or suitable fuel line adaptors/whatever. I'd much rather make these changes to my johnson than get a different motor if at all possible since apart from those two niggles I've been perfectly happy with it.

Happy boating everyone.

Colin
 

Scaaty

Vice Admiral
Joined
May 31, 2004
Messages
5,180
Re: 4hp Johnson remote fuel tank and prop change

Its a gravity feed...no pump....and I doubt you can do much about the prop. Better off sell it and buy something bigger.
 

steelespike

Supreme Mariner
Joined
Apr 26, 2002
Messages
19,069
Re: 4hp Johnson remote fuel tank and prop change

What are you pushing with your motor?
Just for future reference generally speaking when you increase pitch and/or diameter it makes it more difficult to push a boat.Your little motor is most likely propped right because almost any boat would tax it to its max.
The decision for a prop change depends on boat speed,and engine rpm at wot.An engine should reach its max rpm at wot. under a normal load.Generally speaking if a properly set up boat and motor doesn't reach max rpm you would go down in pitch (lower number)If the motor is over reving then usually an increase in pitch is necessary.(higher number).
If your motors antiventilation plate is about even with the bottom of the boat then you are most likely set up right.If you want more detailed info on props go over to the prop forum here.They may be able to tell you any props that are available.Though you will need to know your present max rpm before you can figure a change.
You may be able to borrow a tach if it is to help figure if you need a new prop.
 

colinc

Recruit
Joined
Jan 9, 2007
Messages
3
Re: 4hp Johnson remote fuel tank and prop change

Hi Steve

Up to press I've been using the 4HP Johnson on a 14' plywood dingy - and for that it's pretty good. But what I'm pondering doing for next season is attaching the motor to an outrigger canoe in the hope of getting a little more speed with a view to extending my daily cruising range.

The romote fuel tank query is also about increasing my speed (and therefore range), but also about lowering the overall centre of gravity to make an outrigger as stable as possible.

It's not more power that I'm after... I'm a big fan of minimalism when it comes to engine size but I'm also physically disabled and my 4HP johnson is about as much as I can lug around. What I'm primarily after is speed and distance on a very tight budget. I've explored all the marches and inlets in the immediate vicinity of my mooring and would very much like to get down to the southern end of the island and back in time for tea. With a 4HP engine pushing a 14' dinghy it'd just take too long.

Colin
 

itstippy

Chief Petty Officer
Joined
Jul 17, 2003
Messages
548
Re: 4hp Johnson remote fuel tank and prop change

If you consider your current combination of a 4HP on a 14' plywood "pretty good" then you will be amazed what it will do on an outrigger canoe. That "thrashing" you are experiencing now is called "slippage" from the motor trying to push too much weight and hull resistance. On an outrigger canoe the "thrashing" will stop and you will move forward instead (less slippage). You will also cover a much greater distance before refueling.
Picture yourself paddling the 14' plywood. Now picture paddling an outrigger canoe. Big difference. But before you buy an outrigger be sure to take if for a test run first - with your 4HP.
 

colinc

Recruit
Joined
Jan 9, 2007
Messages
3
Re: 4hp Johnson remote fuel tank and prop change

Re: slippage

You make a very good point there. The outrigger I have in my head would be a little on the large side (as outriggers go) but even so; both the wetted area and the weight would be a good deal less than I have now and unless I'm very much mistaken the longer the hull is the more inherent speed a boat has.

Thanks very much for the pointers!

Colin
 
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