Avon 5.4 SR with twin 40's performance?

[boomersails]

Hi,

Just bought an Avon SR 5.4 with twin Mariner 40's. One is in need of rebuild. I was considering putting a single E-tec 90 on but am curious about the performance data with the twins.

Does anyone know how this boat performs with twin 40's. The Mariners are light and I like the redundancy but I want the boat to be FAST!

Thanks!

[Chris]

If you want the boat to be fast then I would fit the e-tec you will also halve your fuel bill !

[benjytaylor]

Hi, I used to have twin yamaha 40's on my 5.4m SR. There are issues with twins. Firstly, doubled the fuel cost to a single fourstroke/etec ( I have a 75hp Merc 4Snow) . Double the service cost etc. Don't get to excited by the possible speed, you will get 31knts top even trimmed up ( the Merc does 35.5kts ). Then you will have serious chime walk issues If you don't put counter balance weight in the front. I had to get nearly 40kg balast grp'd into the front.
I found the perfect balance using sandbags to start with. The twin setup is great for redundancy and loss o steering ( if you have hydraulic baystar or seastar instead of teleflex. You can clamp the ram at the rear and drive back on just engines )
All in all, it's a bit of an effort changing the boat back to a single, because your boat was probably an original fire rescue craft the transom might have some rot problems converting it to a single. Plus if you have the balast in, fitting a fourstroke could work great.
If your worried about lack of power, my 75hp fourstroke is faster than my 80hp yammie 2S and tohatsu 90hp 2S.

[boomersails]

Thanks,
Were you able to plane on just one of the 40's?

[boomersails]

Thanks guys,
benjytaylor, the transom appears fine. I havent drilled into it but I have seen no evident signs of compromise. Besides there being 8 mounting holes instead of four are there any other reasons you suspect there may be transom rot on the fire rescue boats?
When you converted did you have to move the drain hole off centerline? And since there is no notch on the transom I figured I might need to go with a 25" shaft. The 2 mariners I have are light, what did your yammies weigh?

[benjytaylor]

The yammies are bang on maximum weight for the craft's transom 164kg. Roughly 82kg each, plus the heavy original steel a frame which stresses the transom ( as some people tow of it too ) yes the boat will eventually plane on one engine 13-15knts required to get on the plane.
The hole would need o be moved to the side and the original capped off, of course the 8 holes also need to be capped. Useful if you have a local shipwright to do it for you or tackle it yourself.
The transom will be cut lower to support the twin setup and there is no reason to change that as I have 2 pricing twin seariders ( 1 has been left and one has a brand new transom )















The blue boat has just been capped and has a long shaft yammie 80hp. The cavitation plate is not exactly spot on to the bottom of the transome but does the job. As you can see the other searider had a whole complete new transom as the old had hidden demons and completely rotten

[benjytaylor]

The Merc has also jet ha new toobs fitted this weekend by ribraff :-)

[benjytaylor]

Also if you look at the photo of the Merc you will see I had a bowlocker grp'd in with the 40kg grp'd balast at the bottom.

[benjytaylor]

Some photo's of the searider with twin yammies before it's refit

[njensen]

what is fast ?

I get 31-33 knots @ 5200 rpm with a 90 HP E-Tec on my SR4.7 (15" alu prop).
Inititally i tried a 17" alu prop but it only gave me 4900 rpm wot.
33 knots is fast enough for me on a 4.7m rib...

[Bushrider]

Originally Posted by njensen

what is fast ?

I get 31-33 knots @ 5200 rpm with a 90 HP E-Tec on my SR4.7 (15" alu prop).
Inititally i tried a 17" alu prop but it only gave me 4900 rpm wot.
33 knots is fast enough for me on a 4.7m rib...

Hi ,
Is there something wrong with your setup ??
That 33 knots are fast enough is one thing but I would assume that the max speed with 90hp ETEC on the SR4.7 would be higher !
My old Yammi 90 2stroke pushes my SR5.4 with big Osprey console , 90 liter tank + myself with appr 120kg up to 38knots (GPS) with a standard Yamaha aluminium prop 19 pitch .
Cheers - Jürgen

[benjytaylor]

Originally Posted by Bushrider

Hi ,
Is there something wrong with your setup ??
That 33 knots are fast enough is one thing but I would assume that the max speed with 90hp ETEC on the SR4.7 would be higher !
My old Yammi 90 2stroke pushes my SR5.4 with big Osprey console , 90 liter tank + myself with appr 120kg up to 38knots (GPS) with a standard Yamaha aluminium prop 19 pitch .
Cheers - Jürgen

About right I'd say for a commercial 5.4 with a 90. I have A tohatsu 90 that gets 31ish a yammie 80 that gets 32. The larger merchant gets 35 fully trimmed up. I wouldn't expect to much more and hold on tight for that chine walk

[njensen]

Speed is not so much about HP. It's more related to prop, gearbox, drag and the engine max rpm.

Since my engine is turning 5200 rpm wot @ 32 knots... it can't be too wrong. According to my calculations it gives a prop slip of around 0% (and 5200 is max for the 90 HP)

I might be able to raise the engine 2 holes, and turn a 17" prop instead. That would give around 36 knots assuming no slip. Actually i'm going to try to raise the engine, but that's just to optimize fuel consumption.

My old Suzuki Dt65 gave me 36 knots using a 16" alu prop and 5800 rpm.

I really don't care about top-speed, i need acceleration and fuel performance.

[ooglewoogle]

Can anyone give me a rough estimate of how much fuel I would burn with the twin Yam 40's in this thread? Still mulling a Sr5.4....and I could do with thet win redundancy up here....but have no clue as to the fuel costs.

Good to hear it planes ok with one.

[njensen]

it's a guess.... 1.3-2 liter per NM...

my SR4.7 + 65DT Suzuki used about 0.9-1.1 liter per NM.
with my new E-Tec 90 HP it's more like 0.6 liter per NM.

[benjytaylor]

3ltrs per nm

[benjytaylor]

Originally Posted by benjytaylor

3ltrs per nm

I travel approximate 10nm every day on the searider's- 1. 4m searider yammie 40 : 15ltrs. 2. 5.4SR merc 75 4S : 8- 9ltrs 3. 90 twatsoo : 20ltrs. 4. 80 yammie 2S 18ltrs.
I hope that helps.

[benjytaylor]

Sorry forgot to mention that's cruising @ 20-25knts for about 8nm and the other 2nm @ 6knts