Engine too big?

[bazza]

Hi guys,
Been reading the posts but cannot find an answer to my query.

I have a Avon Searider 4.7m rib. It had a Suzuki dt75 on it and I used to take 4 divers, 4 sets of kit and 4 spare cylinders. It had a 13X19 prop and it would plane and cruise at 20-25kts depending on sea conditions.
Anyway..... the Suzuki gearbox failed and it was always hard to start so I replaced it with a Honda BF75 f/s. I`ve had the rhib in the water and it runs fine although it only had 2 divers and 4 sets of kit.

My question is... Is this engine too heavy for this boat? And if so, is it therefore dangerous?

I know the engine is heavier than the Suzuki but what if I limited how much gear I carried would that make a difference? Or is the difference negligible?

Lots of questions I know, and maybe the answers are obvious, but they`re only easy if you`re in the know.

Any advice gratefully received.

Thanks in advance.

[chewy]

If you have a look on Avons website you should find the details for the max engine weight etc.

Welcome to the forum.

[Poly]

Originally Posted by bazza

I know the engine is heavier than the Suzuki but what if I limited how much gear I carried would that make a difference? Or is the difference negligible?

I am sure Nos/Doggypaddle/RogueWave or one of the other searider gurus will be able to provide some specific comments but its not the effect on "total load" you need to be worried about - but rather the weight on the transom and the fact that all that weight is right at the back upsetting the balance.

Just how heavy is the new engine? I am guessing something silly like 150 kg. Moving gear forward will help rather than taking gear out. In terms of transom damage it may depend as much on your driving style (and if you like to get airbourne) and how its supported on the trailer as anything else.

[bazza]

Thanks for the quick responses so far guys.

From what I can find the engine is 136kg. If it`s just the balance to worry about then we have plenty weight in the dive gear to move forward to couteract the engine. I try and not get "airborne" as this tends to have the gear bounce about, and at £2000ish a set I don`t think I`d be too popular.

I had a look at the Avon website but couldn`t find anything listed about my rhib. Maybe it is too old?

Any further advice appreciated.

[samt]

http://www.avon-workboats.com/page/sr47m

Doesn't state max engine weight. But they will have been designed for 2 strokes, so your honda may be too heavy. You could try ringing avon if nobody on here has the answer.

Edit: Just noticed the one in the photos looks like it has a Mariner 60 fourstroke, which weighs in at 120kg, so not far off your honda

[Poly]

Originally Posted by bazza

I had a look at the Avon website but couldn`t find anything listed about my rhib. Maybe it is too old?

its here: http://www.avon-workboats.com/page/sr47m
no weight specced. max 70hp, but twin 30 4str's would be close to weight you have, so if it were me - use with common sense keep an eye on transom for cracks and fix/reinforce early...

[chewy]

Max engine weight forthe 5.4m is 170kg so its gonna be less than this.

You could try reinforcing the transom yourself for peace of mind?

[bazza]

Thanks again guys. I have already got a sheet of metal(which used to be a metal step in a factory) on the transom and the 4 bolts go through this as well as the transom but I will heed your advice and keep checking for cracks.

So far the advice seems in my favour. I was more worried about it being dangerous. I`m not one for wanting rescued by the RNLI for being a prat.

[bazza]

Doing more searching on the net I have found that there seems to be a contradiction in the weight of the Honda bf75. More sites seem to say it s dry weight is 163kg and not 136kg as I had previously found.(may have been a misprint on the site I found as it seems just 2 numbers are transposed).

I have emailed Avon to see what the max weight is for my Rhib and will see what they come back with.

[PeterM]

I've got 115 KG on the back of a 4m adventure. I thought the searider were built tougher - so I reckon on a 4.7 youd get away with it as long as you are not going for 4 ft of air every other wave ! If its 136 defo - if its more you shoudl be oK with suitable extra support.

[Nos4r2]

Looking at the specs for the 5.4 compared to the 4.7 I get the impression the 4.7 is the same boat as the 5.4 but using a shortened mould. The Honda might not be ideal as it's bloody heavy, but I doubt you'll have any structural problems.

4.7 Specs:-



5.4 Specs:-

[bazza]

Got an email back from Avon.....

There is a maximum 70hp for the SR4.7m Searider.
The maximum transom weight would depend on
if it is a long or ultra long shaft.

The long shaft is standard for this boat.

Maximum Transom weight
Long shaft 130kg
Ultra long shaft 150kg

Has anyone got anything to add now with this info? Considering the engine is a long shaft and weighs 163kg.

[doggypaddle]

AS NOS pointed out the avon is a strong boat and the weight is unlikely to cause any structural problems, and the power of the engine isnt excessive,
however they were designed for twostrokes and i would say the handling wouldnt be exactly as avon designed with 163 Kgs on the back.
maybe if you compenate by moving other weight such as fuel tanks forward it will sort out the handling, but keeping an insurance company happy in the event of a claim is another matter, and this would be a major concern if it were mine.
If you can get your insurance company to cover it, and get it in writing after explaining its X% overweight and X% overpowered, once your used to the different handling i cant see it being a problem.
someone else may know different?

[matt h]

Doggypaddle is right, to me this set up is way to heavy, also the insurance would be totally null and void, as your carrying divers etc this really should be a major factor in your next decision, i wouldnt put a 90 f/s on a 5.4 as its on the limits, definately not on a 4.7,
Avon have already said you have exceeded the hp, really you need to change the engine or not buy it what ever,
Tohatsu 60 /70 or Yam 70 would be ideal

[Turbodiesel]

I think its far to heavy, you will have so much water coming over the transom as it will have almost no freeboard (when sitting in the waater not underway) Also the engine could suffer if its sitting deep in the water. I think you also might have problems if your in a heavy sea as the boat will be so heavy at the back.

[Downhilldai]

Originally Posted by matt h

Tohatsu 60 /70 or Yam 70 would be ideal

Agreed
.
or a Soozook DT65
.
.
.
A mate of mine did have one with a Yam 90...........

[bazza]

Thanks for all the responses guys.

I still have my Suzuki DT75 and I think the starting problem was a bad negative battery lead as we found this when replacing the engine with the Honda. I think given the advice from forum members I will put the Suzuki back on. I have had the gearbox repaired and I now have a spare as well.

Anyone want to buy a Honda BF75 F/S 2002 complete with controls and new prop?:-)


PS. I take it the extra 5HP of the Suzuki over what Avon recommend will not cause any problems....will it?

[chewy]

Nah it'll be fine.