Multiple Bolt Holes in Transom

[Steve509926]

I need advice on whether it is advisable to drill multiple bolt holes in the transom to attach the outboard.
My outboard is sat on a 20mm packer, which is drilled and bolted to the transom. In a lake or a river (Dart) it's ideal. Very little cavitation, only slight cavitation in sharp turns.
Out on the sea with a following swell and 0.75 mtr waves, loads of cavitation, bubbling and burbling away.
My thoughts are to drop the OB to transom height so the prop is deeper. Would this be OK? and if so, is it OK to have so many holes in the transom?

[beamishken]

Cant you just drop the engine via the holes in the bracket or are you in the top holes now?

[Steve509926]

I'm in the bottom holes now. Tried to drop it and use the top holes but there is approx 5mm difference. I think the centres between the top and bottom holes are 25mm, could be wrong - might be a bit less or slightly more.

[beamishken]

There should be an adjustment range of about 80mm up or down if the bolts are in the top holes then your as far down as you can go. But its unusual for the top holes to be too high there more often too low

[Fenlander]

An idea another poster mentioned a while back was to drill holes oversize and seal in a stainless sleeve to make the best job of protecting the transom from water ingress around bolts..... assuming that was your worry.

If it's just re the looks or stigma of holes in the transom it never worries me... do what I need for the outfit to work best.

But I'm puzzled you can't find the sweet spot for all uses. In around 10 SIBs I've owned I've found one height and that's it sorted.

[Steve509926]

Hi Mr F, I hoped that I'd found the sweet spot when I was in Ullswater. As I say it's great on lakes and rivers, but yesterday on the sea between Dartmouth and Hope Cove it was losing a lot of grip in the swell and waves. I think the transom sits high in the water probably due to the large rear cones on the tubes.
Im not bothered about the look of two holes, it's more would it compromise the integrity of the transom? If it doesn't, I may router a slot 20x10 so that I can adjust the position when leaving an estuary or river out to sea easily.

[Fenlander]

Is your transom one that looks like GRP with a wooden core?

[Fenlander]

BTW I'd be inclined to do sea trials re OB height and once its at a point ventilation stops leave it there... that will do no harm on the lakes.

[Steve509926]

Yes it's GRP with a wooden core. Back out on Thursday on the Solent, so I'll see what it's like then, although looking at the sea today it was very calm.

[Fenlander]

It is one of the air floor challenges they can be a bit unpredictable like this. The offset for easy assy and light weight.

[Steve509926]

True, but it doesn't detract from the pleasure of being out on the water ��

[GuyC]

Originally Posted by Steve509926

Hi Mr F, I hoped that I'd found the sweet spot when I was in Ullswater. As I say it's great on lakes and rivers, but yesterday on the sea between Dartmouth and Hope Cove it was losing a lot of grip in the swell and waves.

It's due to the difference in the density between sea water and fresh water.

[Steve509926]

Originally Posted by GuyC

It's due to the difference in the density between sea water and fresh water.

That makes absolute sense.

[jeffstevens763@g]

i had the same problem with my excel 435 i set my cav plate at 10mm below the keel but it cavitated badly so ive dropped mine straight on top of the transom which is 50mm below the keel now. it is correct that you are more buoyant in seawater but you will find the tubes extend way back giving a lot of lift at the transom hence the rule of cav plate level dosnt seem to work i have yet to trial mine so i will report on my thread when i do.
re the holes just fill with sikaflex they will be fine thats all ive done

[Steve509926]

Hi Jeff, I'll let you know how I get on tomorrow with the OB on sat on the transom on my Excel SD360. As a matter of interest, what weight is your OB and are you normally 1 or 2 on board.?

[jeffstevens763@g]

Originally Posted by Steve509926

Hi Jeff, I'll let you know how I get on tomorrow with the OB on sat on the transom on my Excel SD360. As a matter of interest, what weight is your OB and are you normally 1 or 2 on board.?

Steve outboard is 62kg I'm a big bloke plus I had camping kit too plus 65 ltrs of fuel weight wise I'd say three up.

Cheers

[Steve509926]

A similar weight to when I experienced the problems in the swell and waves. I had 3 adults, dog, 24 ltr fuel, usual bits and bobs for the day and a 52kg OB. I wonder if it's similar with all Excel's?
Looking forward to seeing the difference tomorrow.

[jeffstevens763@g]

Originally Posted by Steve509926

A similar weight to when I experienced the problems in the swell and waves. I had 3 adults, dog, 24 ltr fuel, usual bits and bobs for the day and a 52kg OB. I wonder if it's similar with all Excel's?
Looking forward to seeing the difference tomorrow.

I'll be interested to here, another guy on here has the same boat as me with the engine straight on the transom with no cav problems if yours performs I'd stick my neck out and say it's the geometry of the transom to hull and the fact the tube extends well back that keeps the engine height on the plane. To be honest I've never had to pack a sib engine up on other makes

[Steve509926]

Managed to get out today and what a difference it has made dropping the outboard onto the transom.
Launched at Bucklers Hard, gentle cruise up and down the Beaulieu river before venturing out onto the Solent headed up towards Calshot and Southampton water. WOT and not a murmur, no cavitation whatsoever but the sea was relativity calm, so not much of a test. Beached, had lunch and headed back. The weather and the tide had changed, now that was a test! 1 mtr waves, 10 knot wind and a swell, hardly any cavitation and barely any loss of grip. Now I know at what height on the transom the outboard performs at its best. Now I'm happy.

[jeffstevens763@g]

Spot on Steve thanks for the update