We are re-powering a 68 bristol 33 LWL 25' displacement 11500# w/ atomic four, it is a very easily driven hull form. We were thinking of the BD 722 or the Beta 16. The engine is mounted under the cabin sole. The distance from the top of the engine bed ( where the hard mounted atomic four sat ) to the bottom of the engone bed is only 7 inches, will we need a shallow oil pan? The old A-4 is long gone, so I don't know what transmission it had. Exhaust is another issue, the old exhaust was dry and water jacketed all the way to just before the transom then it mixed and discharged as wet exhaust, it is not reuseable, rusted out. Thinking water lift out the transom or possibly out the side? We would appreciate any thoughts on engine/options, is the Beta 16 enough for most conditions? instillation and prop selection ect....
I think that you will need the shallow oil pan as your measurement of 7" down from the engine beds leaves you with marginal clearance. The shallow pan will make the whole installation easier and you wont have to worry about a steel oil pan very close to any water in your bilge. The shallow sump on all the mini series engines is cast aluminum.
I have run a prop calc for you using both the 16 and the 20HP engines. The BD722 (Beta 20) will give you better performance and the prop called for is a 12"x9" three blade. This should give you 6.3 knots with ease (I got the boat specs from the Bristol Owners website). You will have to make a modification to the cabin sole as the new engine will be about 2" taller than the Atomic 4. The A4 had a direct drive transmission with no drop center and all the new transmissions have a drop center of about 2.5".
I recommend using a modern waterlift muffler system with a gooseneck somewhere at the stern of the boat. You will also need to use an anti-syphon valve. I personally hate side exiting exhaust systems because of a number of reasons. The main one is the noise when the exhaust goes under water and then lifts out. If you make the exhaust outlet high enough to avoid noise, you risk filling your dingy with water from the exhaust water. 2nd is the possibility of fumes in the cockpit.
The new engine will fit on your current engine beds and will line up with the shaft. We can help with advice on the rest of the repower project as well.
Thanks for the info, it is very helpful. What kind of numbers did you get with the beta 16 when you ran your prop calculations? I am also concerned that our biggest challenge will be the exhaust system, there is very little room under the engine for a waterlift and an up hill run of 13+ feet. I agree with you, a transom exit is most preferable, but installation is going to be tight no matter what route we take. Any insight on exhaust install details and pros/cons of the beta 16 or bd722 for this application would be appreciated.
The Beta 16 should just get you to 6Knots in good conditions with no reserve. As far as installation is concerned, it is the same mounting and height as the 20. There is $500 at full list price differance between the 2 engines. The Beta 16 will be adequate for most conditions but will not give you much reserve. The 20 will provide some reserve and more relaxed cruising. You are looking at 1/3 gallon of diesel being used in normal cruising per hour, 1/2 gal if you push it.
The exhaust will pose a few installation challanges. Use 2" hose and a full flow transome fitting to minimise back pressure and the length will be ok ( we actually keep these fittings in stock along with a good gooseneck). You should be able to get a waterlift muffler to fit behind the engine, if you cannot, it is easy (but time consuming) to build a custom one to fit the space that you have. I have seen custom waterlift mufflers with a slot to allow the shaft to fit between the port and stbd side and about 2 " of muffler joining the 2 sides of the muffler under the shaft.
I hope that this has helped a bit. Regards, Stanley
We are going to go with the 20, the reserve would be nice. The shaft log inside diameter is only 1 3/8 (the atomic 4 had a 3/4 inch shaft) I think I can get a cutlass bearing in the tube that would fit a 7/8 shaft, is 7/8 OK with the 20 ? We may have to go with a high rise exhaust elbow, is it also 2 inch ? One last question, what gear reduction would you suggest and is the same as you used for the prop calculations ?
I used 2:1 as the gear ratio for my prop calc. You can get by with a 3/4" stainless shaft but it will be at its limits. It will be much safer to go up to 7/8" and it should be no problem to get the correct cutless bearing and stuffing box.
We will definately supply a 2" exhaust elbow in view of the length of your exhaust system.
You will need at least a 14" two blade propeller to be able to get the engine power to the water. I am not sure that your boat can accomodate that large a propeller. I suggest that you confirm what size propeller you can accomodate (please remember to allow for 15% tip clearance to avoid prop wash noise inside the boat) and then speak to the folks at PYI as they know their propellers better that anyone else, and the large flat blade area of a Max prop has different characteristics to stabdard fixed blade propellers.
The standard prop calc comes up with a 15"x10" two blade or 14"x8" three blade. Within reason we can adjust the sizing to accomodate a smaller diameter propeller and still keep enough efficiency to give you good drive and power.
