I joined www.boatdiesel.com for a year when I was rebuilding the engine my 27 Express Cruiser. They have a couple of chat rooms that really sharp guys weigh in on regularly and they have an excellent prop calculator that was instrumental in my getting my prop perfect for my boat when I went from 260 hp to 410hp. I agree with Victor's points. If the maximum rpm is low you will generally need to reduce the pitch and possibly even the diameter as well, although that costs lift. I'm gussing there won't be a ton of difference between 18x17 and 17x18. 17x15 or 18x13 should make more of a difference. You didn't mention your top speed and rpm with the old engines. Did the boat get up onto full plane with the 270 horse motors? How fast was it at top speed, and at what rpm? One of the things I read in my research before I finished my job is that very shallow props are less efficient than steeper props. Beyond a certain point, the prop calculator recommended a lower transmission ratio and steepr prop rather than a higher ratio and shallower prop. I spent hours on the calcultor plugging in different transmission ratios, prop diameters, number of blades and pitch amounts. The price for a year's subscription was something like 25 or 50 bucks and more than worth it to me. I found a great place for a new stuffing box on that site reading the chat rooms. Whatever calculator you use, it will need a displacement figure and a waterline length and that particular one on www.boatdiesel.com has a hull type as well. One place I disagree slightly with Victor on is rpm range. I think most stock marine engine camshafts have a designed top end around 4400 to 5000, not the 5500 Victor mentioned. A high performance version might go that high but you probably wouldn't put high performance motors in a big cruiser unless you are somebody that likes fiddling in your engine room a lot (like me). You might decide to go with high performance small blocks instead of low performance big blocks but most folks would probably go the other way, as you did in this case. If you can get your engines to at least 4400 rpm you should be good. In my case, I took a small block and made it big displacement, sort of splitting the difference. The cam I had specially ground for my desired cruising rpm (3400) has a top end of 5000rpm. It seems unusual to me that you could only get 4400 rpm out of your engines when you had the 2.57/1 transmissions in it. The engines might not be putting out their full horsepower potential and if they aren't, you will never be able to get the thing propped right. I think I would do a compression test and double check the basics like timing and carb settings before I did any more work on the prop selection. It's cheap, easy and will give you more confidence when you do pick a prop combination that you have it right. I choked on the price of one new prop. You're buying two. You are increasing your horsepower by about 30%, which sounds like a lot but might not give you as much of a performance imporvement as you think. I increased my horsepower by 60% and got 20% more cruising speed, 25% more top speed and 28% more fuel economy. Great results to be sure, but not as much as one might think with that much extra power. The one thing that everybody who seems to know what they are talking about agree on is this. Whatever prop pitch, diameter and transmissionration ratio you settle on, it's critical that your engines will reach the maximum rpm the maunfacturer lists for that model with a full load in the boat. Anthing less will dramatically shorten the life of the engines and severely limit the fuel efficiency you can obtain. Good luck, and let us know how it comes out!
John --- On Fri, 8/28/09, Love and Luck <[email protected]> wrote: From: Love and Luck <[email protected]> Subject: [UnifliteWorld] 34ss with 454's To: "UnifliteWorld" <[email protected]> Date: Friday, August 28, 2009, 5:49 AM Hello Everyone, I posted a message last year but never got any responses. I'm looking for information once again on my engine upgrade. First a little background.... I have a 1982 Uniflite that used to have 270 hp crusaders, 1.91:1 Velvet drive Transmissions, and 18 x 17 inch props. My boat would only cruise between 12 and 15 knots. I decided to upgrade my tired motors and put in a pair of used 350 hp crusaders. The motors I bought came with 2.57:1 Transmissions so I initially installed these into my boat. I borrowed a pair of 18 x 22 inch props from a friend and the boat would cruise at 9 knots and top out at 4400 rpm. At this point I spoke with Sea-cure technologies to find out how 34 ss was set up from uniflite. They told me those uniflites came with the 1.52:1 Transmissions and 17 x 18 props. So, thanks to craigslist, I found a pair of 1.52:1 Velvet Drives and put them into my boat. Took the boat out for a cruise with the 18 x 22 props, topped out at 2000 rpm and 10 knots. Next I took off the 18 x 22's and put back on my original 18 x 17's. Took her out for a run and she did 14 knots topped out at 3200 rpm. So, my question, will going to the 17 x 18's bring my rpms up and my cruise speed up? Is there anyone else out there with a 34 uniflite with 454's? Thanks Pete --~--~---------~--~----~------------~-------~--~----~ You received this message because you are subscribed to the Google Groups "UnifliteWorld" group. To post to this group, send email to [email protected] To unsubscribe from this group, send email to [email protected] For more options, visit this group at http://groups.google.com/group/unifliteworld?hl=en -~----------~----~----~----~------~----~------~--~---
