Clearance figures for engine/hull and prop/hull - URGENT

[carioca1232001]

I dropped in the twin-diesel engines cum gear-boxes into my 32ft. motor-cruiser last Sunday. The boat has had her undersides completely overhauled, i.e., 60% refastened and 40% re-planked. Feels solid as a rock.

Since then I have been lining up gear-box flanges with the shaft flanges, besides optimising prop shaft/strut alignment etc.

Before "bolting everything up", I need to know if the following clearances are acceptable:

1. A 2 (two) centimetre clearance between where the engine/gear-box assembly (flywheel housing) comes closest to the inside hull (shaft log);

2. A 6 (six) centimetre clearance between prop tip and hull planking directly overhead.

Thanks in anticipation.

[Ross M]

I do not know about question one, but question two is covered in some detail by Dave Gerr's "Propeller Handbook":

RPM Range; S/L Ratio; Minimum Tip Clearance

200-500; under 1.2; 8%
300-1800; 1.2-2.5; 10%
1000 and above; over 2.5; 15%
High speed planing craft; over 3.0; 20%

Mr. Gerr notes that these figures are absolute minimums.

Hope this helps some...

Ross

[carioca1232001]

Thanks, Ross, for addressing query n° 2.

Unfortunately, I do not have access to Dave Gerr´s book.

Could you please, therfore, clarify what is meant by S/L ratio and % clearance ?

[Ross M]

Hey ho, C1

2a) SL ratio: Speed to Length ratio; determined by dividing the speed in knots by the square root of the waterline length in feet. Thus, for a boat with a waterline length of 70 feet travelling at 11 knots:
11 / 8.367 = 1.315 SL ratio

2b) Clearance % is tip clearance as a percentage of the prop diameter. Thus, a 30 inch diameter prop requiring a 15% clearance requires 30 x .15 = 4.5 (inches)

Note: Mr. Gerr states that in no case should the tip clearance be less than 2 inches (50 mm).
Also note that RPM is the prop speed, not engine speed.

Ross

[carioca1232001]

Thanks again.

Hope someone is able to answer query n° 1.

[mmd]

Two centimetres clearance at the fywheel housing is probably fine. Even with soft engine mounts it is pretty unlikely that the engine will move enough to make contact, and that is not a particularly hot part of the engine.

Your prop tip clearance seems a bit small, though. What is your prop size/rpm?

[carioca1232001]

Thanks, mmd.

The engine mounts are refurbished ones. I had a lathe-shop weed out the older nitrile rubber inserts and substitute with URETHANE. Engine stringers and support blocks have all been retightened and lock-nuts installed.

Wonder though, if the hull may work its way through the 2 centimetres clearance, as the boat bottoms out after riding out a wave crest ? :rolleyes:

Mind you, with ipê (ee-pay)replanking and n° 10 bronze screws refastening the older planks which originally sported n° 8 screws, the underwater hull feels rock solid.

About the prop: radius = 23 cm. roughly and rpm not to exceed 1300-1500 (2:1 gear-box)

When I am through with these basics and have everything sorted out and installed, I will take you up on your previous offer concerning the more intricate workings of power boats (C of G, C of Bouyancy, optimum gear-box ratio etc. ;) )

[mmd]

Prop clearance should be fine. I thought you were running a bigger, more powerful driveline. It still is rather tight, though. I would carefully inspect the hull area immediately above the prop for erosion from prop wash after your first outing, and periodically afterwards. If the bottom paint appears to be being stripped away, maybe a thin bronze sheet bedded in goop and tacked on the hull above the prop will act as a wash guard for the hull in that area. The other problem with a too-small tip clearance is that the prop vibrates badly. If this happens, you will know it immediately, 'cause the fillings fall out of your teeth. In that case, there is no simple solution - you will have to move the prop father away from the hull.

When your boat can flex two centimetres within thirty or forty centimetres of the backbone structure when in a seaway, I think then would be a good time to head for shore. ;) Seriously, there shouldn't be a problem with that clearance. In most of my design work I specify about ten centimetres clearance, but that is so a mechanic can get his hand under the flywheel. Many boats I see have clearances at the bell housing smaller than I can pass my fingertips between.

If you are really worried about this, it seems that your only option is to move the engine forward to gain more clearance. This, of course, would require a new shaft, which ain't cheap. Maybe a compromise would be to put it in as fitted now, but plan any engine box with enough clearance at the front to accomodate moving the engine forward a few inches should you encounter a problem during trials.

P.S. - Yes I did drop the ball on that question about planing hull theory, didn't I. Bad me - I get so easily distracted. redface.gif Remind me in a few days or a week with a specific question and I'll try to do better than last time. Not right now though - tomorrow I have to begin a technical document (Shipboard Oil Pollution Emergency Procedures (SOPEP) Manual) for a rather demanding client, so I'll not have time for more than pleasantries here.

[ 02-27-2005, 04:10 PM: Message edited by: mmd ]

[carioca1232001]

Your discourse has been mighty helpful.

I now have reason to believe that I have lost any gut feeling that I may have acquired during my BSc. Eng as regards structural issues :D

Much success tomorrow with your client.

Sincerely