275 Cosworth Mercury - Page 2

Taz · 14-10-2004, 04:47 PM

pretty sure im just going to try moving the trim tabs down abit and move the fuel tank to the front all i can think is it was ok with the xr2 on there now ive got another 200lb on the back it must be the weight

Captain Chaos · 14-10-2004, 04:48 PM

Quote:

Originally posted by Matt
Yeh captain, but your Y-fronts have skids in before you even start cos you never wash them.

i'm Sprung!

Jon Fuller · 14-10-2004, 06:02 PM

Dunno what lower unit they have, but if it's a less 'speed friendly' profile, you could be experiencing 'blow out', however, I would have thought 70 was a little slow to be getting this.

Also, (I'm assuming the lower unit ratio is 1.87:1), 5000rpm on a 25" prop only figures to about 57 mph.

Captain Chaos · 14-10-2004, 06:35 PM

So that merc came off a searay then!

Sorry....thats the most constructive comment all day!

Anyway........dont go knockin the Taz you guys!! he got his motor over pretty goddam cheap!

Jon Fuller · 14-10-2004, 06:54 PM

I'm not knocking anyone, just trying to be constructive, but sometimes working out the numbers can put things in perspective, not all speedos/GPS's are accurate, that's all.

Taz · 14-10-2004, 07:12 PM

Whats blow out johnson?. Well if my gps is wrong i know its faster then my old xr2 cos a friend has got a ring with an xr2, hes boat was faster then mine fitted with my xr2 now my boat fitted with 275 is like warp factor nine compared!

Good luck understanding that sentence.
And i know its not very accurate but the speedo on the boat sits at 70mph.

I did hear that mph on water was different from land is this true or boxxocks?

Jon Fuller · 14-10-2004, 07:22 PM

MPH are MPH.

'Blow out' is when the profile (for want of a better description, how pointed and fine an entry it is) is a little too blunt, and the bullet, or gear case fwd of the prop is too short, and basically, when you get beyond a certain speed, the blunt entry spreads the water so quickly and abruptly, that the water flow is unable to follow the contours of the casing, and digs a bigger hole in the water than the diameter of the gear case itself, this then allows the propeller to ventilate, or cavitate, the revs climb and the speed falls until you are again below blowout speed.

Clear as mud? I'm not too good at explanations, will try to find a drawing, or do a sketch myself.

JF

Taz · 14-10-2004, 08:05 PM

So will lifting the engine stop it digging in the water? or plastic fins bolted to the lower unit? i know i bolted a pair to an old 80 merc to help in planning.

Taz · 14-10-2004, 08:07 PM

Although holding the throttle at the same amount does not appear to slow the boat down and nor does the revs raise with out touching the throttle

Taz · 14-10-2004, 08:40 PM

Engine mounted centre bolted straight into old xr2 holes def 25 picth prop like new. at the rear of boat is lots of steel for ski pole and reinforcing transom fuel tank big battery oil resevoir power trim pump. manual trim tabs. bow tank fitted to front

Ben · 14-10-2004, 08:41 PM

Quote:

Originally posted by Johnson
Dunno what lower unit they have, but if it's a less 'speed friendly' profile, you could be experiencing 'blow out', however, I would have thought 70 was a little slow to be getting this.

Also, (I'm assuming the lower unit ratio is 1.87:1), 5000rpm on a 25" prop only figures to about 57 mph.

And that's being charitable assuming only 10% slip.

Jon Fuller · 14-10-2004, 10:10 PM

Quote:

Originally posted by Ned
And that's being charitable assuming only 10% slip.

Yeah, my twin just about manages 10/11% with relatively low 'X'

Hey Taz, you say it has 'manual trim tabs', where are they set, ie, if you were to put a straight edge along the hull in line with the tabs, would the tail of the tab stop the straight edge touching the boat at the transom?

Ben · 14-10-2004, 11:28 PM

Quote:

Originally posted by Johnson
Yeah, my twin just about manages 10/11% with relatively low 'X'

Hey Taz, you say it has 'manual trim tabs', where are they set, ie, if you were to put a straight edge along the hull in line with the tabs, would the tail of the tab stop the straight edge touching the boat at the transom?

There are more than one of you?

Jon Fuller · 14-10-2004, 11:31 PM

very good! who says the Yanks don't have the same sense of humour

Jon Fuller · 14-10-2004, 11:33 PM

Quote:

Originally posted by Matt
I think the box ratio is 1.6, maybe 1.62. This makes the sums add up a lot better.

Ah, now that'd change things, surprised at that ratio tho' with them being more an 'open fisher' type motor, rather than a super sports.

Jon Fuller · 14-10-2004, 11:34 PM

Well I don't like to brag

Jon Fuller · 14-10-2004, 11:38 PM

they really aren't the quickest thing around tho' TD did some work on Ring 21 (cuddy cabin, like his 21) with a 275 on a bracket, I don't think it did much over 60, imagine that boat with a 2.4 efi (245hp) or a 2.5 efi (265hp) it'd be lively.

Ben · 15-10-2004, 12:09 AM

Quote:

Originally posted by Johnson
very good! who says the Yanks don't have the same sense of humour

No, we just tend to be a bit more reserved than our cousins on the other side of the pond. By the way, the correct spelling is humor. As in harbor, color, demeanor et. al.

Jon Fuller · 15-10-2004, 12:15 AM

Quote:

Originally posted by Ned
No, we just tend to be a bit more reserved than our cousins on the other side of the pond. By the way, the correct spelling is humor. As in harbor, color, demeanor et. al.

Ben · 15-10-2004, 12:19 AM

Quite