samsturdy said..

Regarding the exhaust situation, it also states that the exhaust back pressure should be kept to a minimum, so that means
no under water exhaust as that will dramatically increase back pressure. So, going along with resizing the prop to correct
size to keep the exhaust outlet above water seems the right thing to do.

I thought you would be using a water muffler, Vetus type. That will ave some back pressure.

samsturdy said..
Yes you are right Cisco. The graph shows max torque at 2600 rpm with a fuel consumption of one and a half litres per hour.
Now that's almost the revs they're telling me not to exceed for 20 hrs, so it looks like there will be no harm done sitting at
maximum torque when I get going again.
Regarding the exhaust situation, it also states that the exhaust back pressure should be kept to a minimum, so that means
no under water exhaust as that will dramatically increase back pressure. So, going along with resizing the prop to correct
size to keep the exhaust outlet above water seems the right thing to do.

I wouldn't be changing the prop so the boat goes slower at cruise revs.
If the engine runs at or near full revs with the prop you have then the prop is OK. A smaller prop to stop the stern squatting will mean the prop is too small for the engine and the engine/prop won't perform well nor will the engine be under the optimum load.
If the stern is being pushed lower in the water it is because the engine and prop are pushing the boat faster, causing the bow to lift and the stern to squat.
It is possible that your engine/prop combination is too powerful for the hull. Not much you can do about that now and it can be beneficial in some situations.
I still believe you will be best served by raising the exhaust outlet to get it out of the water when at cruise speeds.

samsturdy said..
I think you've just thrown the proverbial spanner MB. That sentiment had flitted across my mind too as the
engine revved very easily to 2000 rpm. However as a novice in the engine/gearbox ratio/ hull length and potential
damage that can happen, I think I'll leave the judgement to Glen at Porters . I'll take all the info and whatever
he says is the way it's going to be. Yes I realize I'll lose speed with a smaller prop but there's got to be a reason
why such things are worked out and I'm not going to push my luck bucking the system.
It'll be interesting talking to Glen and getting his take on the subject.

My apologies Sam. I missed the post that Porters had advised the prop is too big.
However, you have increased power by a good 40% or more from the old engine to the new. The stern will squat down more with the extra speed you will push through the water so the odds are the exhaust will still be under water. With the reduced load the engine will be under with the smaller prop it might give you a gurgle rather than a screaming banshee. If it doesn't you might have to reroute the exhaust.
I have never met anyone who has done an engine change that didn't result in a number of unforeseen problems.

I've just had a call from diver Dave to say he's taken the prop off and put in the cockpit locker. Porters, here I come.

shaggybaxter said..
Go Sam, good on you, this has the potential to be a big change. Hopefully for the better!
I like that feeling of trepidation and butterflies when you're mucking around with big decisions on a boat, the type of decisions that can alter your "on boat" experience.
It may be prop size/pitch, engine size, water tank size and placement, electronics, sail size/type/weight/cut, autohelm/windvane, sensor calibration, (insert favourite here) . It's all interesting to learn a piece of, as I often think we're like manager's of a small ecosystem in some ways, needing a little knowledge in a lot of areas. Part of the fun in repairs and modifications for me is learning more about that system.

I'm looking forward to hearing the outcome Sam, thinking positive for ya.

Thanks Shaggy, that's so kind. I've been at Luna Park Sydney watching my Grandkids in the national championship dance contest,
they're doing so well.
I went to the boat in the dark last night to retrieve the prop and I'm heading off to Porters in fifteen minutes.
Thank you everybody for your interest. I'll report back on Glens advice and final decisions.

Righto. to finish off this thread. The resized prop ( now 12 x 9, it was 13 x 10.5 ) was put on the boat by diver Dave and I trialled it
yesterday. The noise problem greatly diminished, just a bit of a 'swirly' noise as the stern settles down but quickly fades to a
very standable 'splashy' sound. So, job done.
However....the thing you're all going to be so proud of me for is my use of nswsailor's idea of grabbing the mooring rope from
the cockpit, yes, I did it, and it works a treat. I tried it in Towlers Bay picking up a rope mooring and also my own 'stand up' mooring
in the Creek
Here's how I rigged it. I used 8mm nylon rope with a snap hook on one end, the hook is attached to a stanchion base nearest
the cockpit and the line is lead down the outside of the boat, through the pulpit and back up to the first stanchion which has a
stanchion fairlead attached as do all the stanchions up to the cockpit. Putting the line through fairleads keeps it well out of the
way of foot traffic reducing trip hazard. The fairleads are placed just below winch height should the necessity of winch use arise.
prepare to moor by detaching the hook from the stanchion base an drape it over the top lifeline ready to use. Approach mooring
bouy/ rope in normal fashion but allow the boat to slide past in neutral until the bouy is in grabbing distance ( if you have a'stand
up ' like me you won't need a boat hook ) grab the bouy and just let everything fall back into the water until you reach the 'eye'
put the snap hook through the eye and snap it back onto its own line and drop everything back in to the water. Pull on the 'inboard'
line and the mooring eye will appear at the bow. I now casually walk to the bow carrying the inboard line with me ( while all the other
yachties are applauding my genius ) just take the eye and put it on the cleat, detach the snap hook and walk back to the cockpit
trailing the snap hook outside of the stanchions and reattach to the stanchion base again, pull the inboard line tight and tie it
off and you are set for the next mooring. Easy peasy.