Help With Inboard Shaft Angle

Bit like the Spruce Goose then ,although I am sure this postdates that and the war by many years. Vosper Thorneycroft bought PP jets ,Jets are very weight sensative ,I know it's a bit far fetched but could this be why there appears to be a piece missing from your boat.They are in your hometown,you may even have some mates that worked there why not ask around a bit more about the history.

 

You still haven't told us what is the optimum rpm for your engine, but what we know now are two situations.

1. 80 hp, 2:1 gear ratio and a 16x11 3 blade propeller with rather small blades reaches 10 kn at 4500 rpm with the shaft, keel and rudder configuration in the photos.

2. 80 hp 2:1 gear ratio and likely a 16x15 propeller with a Volvo stern drive reaches 14 kn.

I think the key difference here is the propeller. Sure inclined shaft may reduce efficiency and keel and rudder have more drag than a stern drive, but not enough for a 10 vs. 14 kn difference. Probably your boat has its worse drag around 10 kn.

As I said earlier, the propeller seems to be cavitating. Without cavitation it would not rotate more than ~2000 rpm (shaft speed) at 10 kn and 80 hp. What happens to thrust and efficiency during cavitation depends much on the propeller and it is likely that a propeller designed for Volvo stern drive handles cavitation better than the one we see in the pictures. Also the bigger pitch of the stern drive propeller probably helps in this situation.

In order to get better thrust you need a bigger propeller. Since I don't know the target rpm, I can't recommend any pitch. There seems not to be room for a bigger diameter, thus you need more blade area. Maybe 4 blades as well.

Your current propeller looks like EAR~0.5. You need something like EAR=0.75 to get full thrust at 10 kn without too much cavitation.

 

Current shaft angle with the hull bottom is 32 degrees!

 

You can't measure the angle accurately from a picture not taken perpendicular. I have tried once and got an error of 15 degrees.

But yes the angle looks rather big. It is quite laborious to change the angle, thus I would first try another propeller.

 

Good Morning
Where is this 80hp coming from ? Perkins Prima {Volvo MD22] or similar.

 

Imported the jpg in CorelDraw, drew the angle, rotated it away from the perspective and measured it. Not exact I agree, but certainly not more than 2 degrees error. No waterline visible, so I had to take the bottom as a reference.

 

80hp is on FORD TRANSIT 2.5 engine
if CDK is right and the angle is that steep does that mean that would be my problem?
With regards my Revs i cant tell you much as i dont hav rev Counter
With a transit engine you get your HP at the higher end of the revs being 4000-4500

 

Well, rich99, w/o reliable tacho readings, there is nothing to hang your hat on. Quick check on propeller data tells that your engine:

1/ would not reach full power rpms with the setup
2/ if revving into the fourthousands, it would cavitate heavily

In neither case would it be able to propel those 2 tons at anything like planing speeds. The thrust line from that propeller is pushing her nose into the air and loosing lots of power because of bad inflow conditions (I think CDK is coming quite close with the shaft elevation angle).

Sorry, but you have to redo and do right. First, there is no Volvo drive with 2.0:1 gearing. Second, the aft pad on that hull tells me it has been designed for water jet (originally or as an aftermath). Third, with fresh 80 horses, and propped to reach correct rpms, you have a weight to power ratio of ~25 kg/hp. With everything correct that should bring you to something like 18 kn.

BUT NOT WITH ALL THAT SCRAP HANGING FROM THE BOTTOM! You have two options: either a correctly designed tunnel, or a correctly designed surface propeller. Both require correct numbers for gearing and correct propeller.

 

Good morning Rich
May I make a further suggestion .from the work you have done so far you are obviously quite a handy chap and I think you would manage this quite easily. Remould the aft end of the keel to continue the profile to the transom ,easily done,screwed wooden former, GRP ,fair in and flowcoat.this will eradicate the low pressure area here. Fit an outdrive leg using the new lowered keel line as a datum for the anti cavitation/ventilation plate.This will give you ,with your NEW engine,120HP [less the losses from two sets of bevel gears] enough horsepower to plane this hull.I know this isn't what you want to hear but to my mind is the most commonsense approach

 

It would be nice to know the real shaft angle, but if it is really that steep, wouldn't it push the stern up and thus bow down. Especially compared to horizontal shaft (e.g. stern drive) that certainly pushes the bow up. The rising bow further increases the angle of thrust compared to horizontal, thus making it worse at ~10 kn.

Why wouldn't a stern drive be an option? Due to "water yet" hull form? At 18 kn stern drive should be a very good option for a normal planing boat.

 

If the engine is a NON turbo 2.5l Ford transit engine, the 80 hp may be a bit optimistic.
www.lancingmarine.com/fsd.pdf

 

Bit, we are both "students" in this case.
Since it is an issue not related to this particular boat, I have opened a new thread about general theory and practice (actually, more about my doubts... ) of inclined props here:
http://www.boatdesign.net/forums/pr...ce-vs-shaft-inclination-30481.html#post322314
because I believe it deserves more consideration and would be off-topic to discuss it here.

 

Hi all.
I really wanted to get away from the outdrive leg because as i said it was always going wrong and a lot more maintinace than a shaft. But something worth thinking about Cheers.
It wouldnt be to hard for me to change the angle of the shaft as i have now aquired a v drive Borg warner Box.
what i am in the process of doing is welding up a cradle for the boat and i will bring it up on the scrubbing berth on a high tide. then i can take a few days doing the job. With the boat on the cradle i can cut all the keel off. And I have a V bracket I can put on to support the shaft. At the same time i will sort the rudder out as it wont need to be so Big. What you guys think ?
It is a non turbo. However i am in middle of converting it to a turbo. which involves New 130 psi Injectors different fuel pump so i can turn up the fueling turbo wich is a 3L iveco turbo (Turbo being bigger so it wont quick in untill higher Revs.) / Charge cooler and other bits. Lancing do a 140HP i think which is basicly a souped up ford engine.
Or Beacuse i have this New Borg Warner Box i can just put in alot bogger engine as i will not be limited to space.
I am having a problem identifying the ratio of the Box as half its plate is missing but i will open a new thread for that.
Just out of interest if the ratio of the box is less for example 1.25-1 or even 1-1 could i still use it and get good results ??

 

If you are anyway putting more power and doing rather big modifications, you should definitely go back to stern drive. At 10-18 kn the difference is not that big, but with 140 hp you are going to be around 30 kn with good setup. At those speeds the difference between a good shaft setup and a standard stern drive is easily 5 kn.

A good stern drive should not need a lot of maintanance and should not cause any problems.

1.25:1 or 1:1 gearing is not going to work well with a 4000+ rpm engine. If the propeller shaft is rotating 4000+ rpm, you are not going to get a decent efficiency. At 3000 shaft rpm you can still get a somewhat reasonable efficiency around 30 kn, but you would be much better off around 2000-2500 rpm.

 

Ok so that kinda ration would be more suited for a Ford D engine which is about 120hp at 2500 revs ??