27sport

iTrader: (1)

Hi, I am in the process of converting my 27 mag with Volvo 280's to Bravo Ones as the time has come.

By my research it seems that 16.5" is a good number for the X, but I wanted to make sure ...it seems as though that number will put the new drives quite close to where the old X was with the Volvos which kind of confuses me. I always read that Magnum put the Volvo's a bit deep on the boat and that is why they were a bit slow... is it possible mine were higher than normal? The boat only did 56mph so I think that was right in line with the power I have ( 320 chris crafts flv's) and other Volvo boats. Does anyone have a comparision measurement for 280 to Bravo propshaft -crankshaft hieght difference?

I have one chance to get this right and would value anyones opinions on the matter. The only thing I want to avoid is cutting my hatch, so I dont want to go too high. Going with stainless marine manfiolds and risers updgrading from the huge log manifolds on the motors now. Looking to gain some top end obviously but cruising efficiency and lower planing speeds are just as important to me.

Thanks!


Paul
27sport

24degrees

Paul,

I can give you some measurements from my boat...it works really well.

E-mail me at [email protected]

Aaron

27 MAGNUM

I am at 17.5 and I have not found props that get me on plane in the slop yet

MDonziM

17.5 is a little aggresive. I assume you are running a 4blade prop? My x is 17 1/8 and I turn my bravo 1's inward, made a big difference.

CIG3

17" is the magic number. I've done (1) 27' Magnum and (2) 28' Cigarette's with this number and have good success.

27sport

iTrader: (1)

Thanks all ! This is really helpful. How much does tank palcement affect this, I have 4 tanks : 2 50 gal in the seating area , and 2 35 gal in the berth but they are not used. Some boats have only the tanks in front I think. What affect would this potential balance change have, if any on the X?

Based on the feedback I am thinking 16.5 or 17 is probably best. I have always ran 3 bladed props , I dont have a lot of experience with 4 bladed props .

Paul

scippy

iTrader: (3)

Paul,

To me, even with the X at a conservative height wouldn't the C of G swing towards the stern considering the 2 50's of fuel (under the cockpit sole) and twin engines past the mid point?........I'm new to this and I often wondered how bow lift is affected by a higher or lower X?.....I guess you always have those 2-35's for some foward ballast if you need.

MDonziM

I dont think the C /G would be affected too much. Most sports have 2 main tanks in the cabin. I assume you have small block motors? I would make sure the tanks in the cabin are tested and safe for use in case you want some weight forward. My boat has a 42 gal tank forward of the cabin that I use as reserve and balance but I have big blocks which is more weight aft. Generally speaking a higher X dim will make the boat run a little flatter. A 4 blade prop also creates more stern lift than a 3 blade. I would aim for 16 3/4" X if you want to run 3 blades and 17" for 4 blades. I forget how the x-dim is calculated but my propeller shaft is 5 1/8" below the bottom.

88bullet

iTrader: (1)

i can take measurements from cig92's magnum. he knocks on the door of 90mph with his warmed up 500's. he has volvo plates with bravo gimbles on his boat. trick setup for sure

27sport

iTrader: (1)

I was going to use those adapter plates but I decided to keep the back clean and reglass. That X looks high!

Thanks all for the advice!

Paul