F21 outboard motors

Engines, Drive trains, Propellers, Steering, Ground Tackle and other mechanical system
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newt2u
Posts: 93
Joined: Wed Sep 11, 2019 11:24 am
Location: UK

F21 outboard motors

Post by newt2u »

I have had some problems with my 40 yo, 2-stroke Mariner overheating on my last two trips and have decided to replace it. I have just bought one of the new Honda BF series engines. The old engine was 4hp and the only other post re F21 engines on this site described a 4hp outboard. I was wondering what other F21 owners use. I decided to go for the 6hp long shaft variant, as it did not cost much more then the 4hp and from what I have read the 4hp has the same single cylinder engine as the 6hp but is just detuned, same capacity, same weight, so didn't see the point in the 4hp, I'm sure the F21 can use the extra power. I was limited to about 1/3 throttle on the Mariner or it would start to steam in the telltale, so don't really know what the full 4hp will do in terms of pushing the F21 along. Interested in your experience if you have an F21.
Rockin - F21 twin drop keels - located Milford Haven
Wintered in Shropshire where I am when I’m not sailing

Lionel7
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Joined: Fri Dec 05, 2008 12:12 pm

Re: F21 outboard motors

Post by Lionel7 »

My first 21 had a 2 HP Honda. Fabulous motor but not enough power for my needs. Way underpowered unless you are going short distance in still air/water. My second 21 has a wonderful 6hp 2 stroke long shaft electric start 90’s Yamaha. Best outboard I have ever owned. Starts every time with one burst of the electric start and plenty of power to make 5+ knots into wind and chop. I love that motor.

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newt2u
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Location: UK

Re: F21 outboard motors

Post by newt2u »

Thanks Lionel, that sounds good. I have fitted and fired up the Honda now but it's too wild here for a sea trial at the moment. I don't have electric start on mine but its very easy to pull start as it has automatic decompression on the exhaust valve. If it does as well as your Yamaha, I'll be well pleased.
Rockin - F21 twin drop keels - located Milford Haven
Wintered in Shropshire where I am when I’m not sailing

hkowalczyk
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Joined: Sat Sep 04, 2010 1:55 am

Re: F21 outboard motors

Post by hkowalczyk »

What prop do you have on it? I have a 4HP johnson and it is way better with a sailboat prop on it.

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newt2u
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Re: F21 outboard motors

Post by newt2u »

It is the standard for the engine type, 200x150mm 3 blade (7 7/8 x 5 7/8in). I don’t know enough about props to know the difference between a regular and sailboat prop. I assume something to do with speed, i.e. does the hull plane or not under power. So do I need a bigger or smaller pitch?
Rockin - F21 twin drop keels - located Milford Haven
Wintered in Shropshire where I am when I’m not sailing

chadrider
Posts: 22
Joined: Tue Jun 10, 2014 5:36 pm

Re: F21 outboard motors

Post by chadrider »

My first motor was an 8HP two stroke 88 Evenrude that was on the boat when I bought it. LOTS of power but terrible fuel economy and a bear to keep happy (ethanol fuel and all). Second motor was a 5HP Mercury 2 stroke. Really awesome engine: light, powerful, smooth. BUT, it too suffered at the hands of ethanol fuel. Third motor is a Nissan (Tohatsu) 4HP four stroke which starts reliably (assuming I start it at least once a month), is wonderfully quiet, doesn't smell, gets twice as many hours from a gallon of gas, and has adequate power for any situation I have encountered. My biggest complaint about the Nissan is that I want it to have a higher thrust prop since it has top end I cannot use ( with more throttle the revs go up but the speed does not) and is a little anemic in choppy situations.
Chad Rider
Rumford, RI
02916

Freedom 21 "Frisson"
1984, Deep Keel, Gunmount.

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newt2u
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Joined: Wed Sep 11, 2019 11:24 am
Location: UK

Re: F21 outboard motors

Post by newt2u »

Hi Chadrider, yes I agree, my 4 stroke Honda is much quieter and less smelly that the old 2 stroke Mariner I replaced. I am still running the Honda in but was out today and had it at 50-60% throttle for a while, pushing against wind in chop, I was doing about 5 knots, so I am very pleased so far. Your top end loss may not be prop related. With a clean hull the hull speed of the F21 is about 5.5knts or 1.34*sqrt(LWL). You may get a little more than that with more power but not much and it will waste fuel. I doubt your 4HP is enough to get the F21 onto a plane.
Rockin - F21 twin drop keels - located Milford Haven
Wintered in Shropshire where I am when I’m not sailing

chadrider
Posts: 22
Joined: Tue Jun 10, 2014 5:36 pm

Re: F21 outboard motors

Post by chadrider »

newt2u wrote:
Mon Aug 24, 2020 9:57 am
Hi Chadrider, yes I agree, my 4 stroke Honda is much quieter and less smelly that the old 2 stroke Mariner I replaced. I am still running the Honda in but was out today and had it at 50-60% throttle for a while, pushing against wind in chop, I was doing about 5 knots, so I am very pleased so far. Your top end loss may not be prop related. With a clean hull the hull speed of the F21 is about 5.5knts or 1.34*sqrt(LWL). You may get a little more than that with more power but not much and it will waste fuel. I doubt your 4HP is enough to get the F21 onto a plane.
Ahhhhh, but on a few wonderful occasions my gunmount spinnaker was! Actually, the 8HP Evinrude would dig a big hole in the water if asked but, as you say, at the expense of many gallons of gas. I think that with a slightly grippier prop I could get better response to throttle on the low end without losing any top end (which I don't have). Probably at the cost of a slight decrement in fuel economy over long motoring periods which is something I very seldom do.
Chad Rider
Rumford, RI
02916

Freedom 21 "Frisson"
1984, Deep Keel, Gunmount.

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newt2u
Posts: 93
Joined: Wed Sep 11, 2019 11:24 am
Location: UK

Re: F21 outboard motors

Post by newt2u »

Ahhhhh, but on a few wonderful occasions my gunmount spinnaker was!
...yes I'm still waiting for that, only had the gunpoint spinnaker up once and it was in very light air. Out of interest, what was the wind speed required for that?
Rockin - F21 twin drop keels - located Milford Haven
Wintered in Shropshire where I am when I’m not sailing

hkowalczyk
Posts: 26
Joined: Sat Sep 04, 2010 1:55 am

Re: F21 outboard motors

Post by hkowalczyk »

It looks like your pitch is already the lowest it goes for that engine, and the "high thrust" props only start at the 8 hp level. The blades have more area as well as the pitch change. Maybe they are interchangeable. On the Johnson line the 6hp and 8hp were basically the same engine.

I've also got a 9.9 sailmaster that the boat came with, I could probably haul a skier behind the boat with that one! I figure if I ever go out on big water with the boat I'll take that one in case I need more power to make it through currents and wind.

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