To re-power or not?

Unknown · 30 September 2019, 20:08

Evening all,

I’ve recently bought an Avon sr5.4 with a 2004 60hp Mariner Bigfoot 2 stroke hanging off the back. It’s my first ever rib and after getting on a mates one for a day out a little while back I knew I needed to change from speed boat to rib.

Went out for my first test run in it today, topped out at 27mph on a completely flat and calm lake. Got on plane well and very quickly too but the bow never really lifted. It was such a majestic ride I could have nodded off. My mates one is the complete opposite, it has a 96 75hp mercury 2 stroke on it and reaches nearly 40mph. It will stand almost on its end when given WOT from standing start, even when trimmed all the way in.

Is mine under powered? Should I be looking to re power her?

Bushrider · 30 September 2019, 20:27

If this is your first boat I would not change immediately to another motor . You get easy on plane and its just the top speed which is missing . I would spend as many hours as possible at sea to get more experience with the boat .
Wait until you are out and the sea is not flat and calm . Then top speed does not count anymore .
I took my recently acquired SR6 with 2x Yamaha 60s out at the solent recently . I was convinced before I ever been on the boat that it will be probably underpowered - 2x 60 is the minimum recommended power by AVON for the SR6 .
I was very positively surprised that the Yamahas pushed the boat very well . I also got quick on plane and she tops out at "only" nearly 30kn .
22 to 25kn is a nice cruising speed - and this also when conditions are changing and some other boats have to throttle back already .

brveagle · 30 September 2019, 20:32

Is the motor at the proper height? Whats your prop/rpms?

My 540 zodiac hurricane has a 90hp 2 stroke and will do just over 35 knots.

Unknown · 30 September 2019, 20:44

It’s not my first boat no, but I am by no means overly competent. My first boat was a 15ft bow rider speed boat with a 75hp mercury on it, wasn’t really stable enough to venture out too far to sea, hence the recent change.

I know huge hp is not required when it gets choppy but don’t really want to be going everywhere at WOT and I’m not sure if 60hp would cope well with a strong tide or small chop?

It’s currently running a 13.25 prop with a 16 pitch. Not sure on RPM yet though as I haven’t a gauge fitted yet.

Dan Gurney · 30 September 2019, 21:26

Quote:

Originally Posted by Unknown

It will stand almost on its end when given WOT from standing start, even when trimmed all the way in.

N.B. This is not a good thing.

Clamchowder

This is my old Humber Attaque 5.3 with an 85 Yam 2 stroke on. Up over the hump nice and quick then riding nice and flat. Top end was prolly about 30 knots (downhill, on a good day). As far as I remember it was plated for 150 or summat daft, but after contacting Humber they say a 90 4 stroke was the biggest new engine you could put on it.

Regards re-powering... if you trip over a suitable motor at a good price (Yam 90 two smoke for example) then fair enough; but they are like hens teeth so don't hold your breath. I also would advise looking carefully at any old 2 strokes before you buy. You dont want to end up with some shonky old bag of bollocks that'll cost you a fortune (mine needed the top mounts doing before I could sell it with a clear conscience, which was a powerhead off job

).

If you're buying brand new then it's a *lot* of brass and you'll probably be limited by weight; the transom plate may say it's rated to 90 but that's probably a two stroke weight.

If it's a bigfoot it's supposed to be high torque I think, so it might swing a bigger prop. See if you can find somewhere that'll lend you a could of option pending a sale.

Unknown · 30 September 2019, 22:24

So I still have my 91 75hp 2 stroke mercury attached to my old speed boat. It’s got an intermittent fault that I’m chasing at the moment before I’m ready to sell it. When it’s running though, it’s a brilliant engine. Very strong and pulls great through the rev range. It’s unknown hours though and 28yrs old. My current 60hp is absolutely perfect, very quiet and runs like a dream. It’s a 2004 model with only 250 hours.

Do I swap them before I sell 🤔

Last Tango · 01 October 2019, 08:43

Was the "Big Foot" not the variant of that engine with a lower ratio gearbox?

If that is the case there is probably scope for playing around with a few different props if you can borrow them. 16" sounds a bit fine if it has a low ratio box.....

Unknown · 01 October 2019, 09:34

Quote:

Originally Posted by Last Tango

Was the "Big Foot" not the variant of that engine with a lower ratio gearbox?

If that is the case there is probably scope for playing around with a few different props if you can borrow them. 16" sounds a bit fine if it has a low ratio box.....

I thought it was the other way round, think standard is 1.6 and the bigfoot is 2.33

Last Tango · 01 October 2019, 10:17

Quote:

Originally Posted by Unknown

I thought it was the other way round, think standard is 1.6 and the bigfoot is 2.33

That would be 1.6:1 & 2.33:1 that makes the "Big Foot" a lower ratio.
As far as I know the "Big Foot" was aimed at use on heavier boats hence the reason for the lower ratio.
I wouldn't rush out spending money on a new propeller but if you could scrounge a loan of something a bit coarser it would be worth trying.
Do you know what your current WOT rpm is?

spartacus · 01 October 2019, 10:18

A Yamaha 90hp 2-stroke would be perfect, but be content with the Mariner for just now. Better the devil you know and all that. These are old engines now, but there are mint examples out there. The Yamaha 75hp is a good call too.

Unknown · 01 October 2019, 10:21

Quote:

Originally Posted by Last Tango

That would be 1.6:1 & 2.33:1 that makes the "Big Foot" a lower ratio.
As far as I know the "Big Foot" was aimed at use on heavier boats hence the reason for the lower ratio.
I wouldn't rush out spending money on a new propeller but if you could scrounge a loan of something a bit coarser it would be worth trying.
Do you know what your current WOT rpm is?

Ah right ok, thanks for that. I'm not sure about the rev range yet as I dont have a rec counter fitted at the moment

Nos4r2 · 01 October 2019, 17:00

Quote:

Originally Posted by Unknown

I know huge hp is not required when it gets choppy

I'd far rather have the full rating for the hull in chop-if you're knocked off the plane it's quicker to get back on and to be able to turn the boat fast and jump on the plane in really bad weather is a massive plus. Hence why my flooding hull is blocked as well.

Last Tango · 01 October 2019, 17:45

Quote:

Originally Posted by Unknown

So I still have my 91 75hp 2 stroke mercury attached to my old speed boat......... 🤔

What prop is on that..?

brveagle · 01 October 2019, 18:11

Quote:

Originally Posted by Unknown

Ah right ok, thanks for that. I'm not sure about the rev range yet as I dont have a rec counter fitted at the moment

This needs to be step one. If you have too small or too large of a prop, that will greatly effect your top speed.

Orwell boy · 01 October 2019, 18:35

Quote:

Originally Posted by Nos4r2

I'd far rather have the full rating for the hull in chop-if you're knocked off the plane it's quicker to get back on and to be able to turn the boat fast and jump on the plane in really bad weather is a massive plus. Hence why my flooding hull is blocked as well.

i have to agree ,i think this is far more important than top speed , i have just repowered my R/C 5.3 from a stunning 60hp plodder to a max for hull honda 100 vtec ,its like nite and day ,but the most striking thing is the improvment in sturdiness when tvavelling thru seas off the deben and orwell off the east coast ,you feel well powered to hold a strong course
Need to be a bit careful if you are being proffesionally main dealer re powered as i ended up new everything and a large bill all of which i think was well worth it ,but did have a bit f a £ shock
Did set up an nmeaine details on my existing plotter which was another bonus

system to get eng

Unknown · 01 October 2019, 19:25

Quote:

Originally Posted by Last Tango

What prop is on that..?

I’m not sure, I wasn’t aware the 2 would have been compatible so I never checked. Will check in the morning though

Last Tango · 02 October 2019, 12:37

Quote:

Originally Posted by Unknown

I’m not sure, I wasn’t aware the 2 would have been compatible so I never checked. Will check in the morning though

Not sure. The 60 Mariner, I think, is a Yamaha. The bigger ones being Mercury but that gearbox came from a bigger engine in their line-up so it might be Mercury.

Phil M · 03 October 2019, 13:26

Quote:

Originally Posted by Unknown

So I still have my 91 75hp 2 stroke mercury attached to my old speed boat.

Do I swap them before I sell 🤔

Definitely worth considering if you trust the engine. Not sure if the controls are interchangeable but it could be a relatively simple swap if they are.

Phil M