Prop slip increase with speed.
 

Has anyone experienced increasing prop slip with increasing speed. I have an '89 Formula 292 with the old blunt nose bravo lowers. I think they switched to the new high speed cases in '90. Either way my prop numbers drop to about 55 MPH which are 11%, then they begin to increase up to 16% at 66-67 mph. I have run both Bravo and Hydro props and both do the same thing. I'm thinking about swapping out the old cases with the newer style. Anyone think that these old blunt cases are increasing my slip numbers?

All of my other boats have always had better slip the faster I go, this is the first time I have experience this.

Yes I know I have posted this also in the Prop section, but I know some guys frequent this area more, so it is double posted.

Are these speeds on GPS or speedometer ? are you over trimming to reach the higher speeds ?

Yes the speeds are GPS. No I am not over trimming. I could not tell you where I am a trim wise, but I know it's not past 8 on the indicators.

Sounds like it could be the lowers. I dont think the original bravo's were meant to see much over 55mph

That's what I think as well... I'm good to 55 mph then the slip begins to really drop. When I look at the guys with 415 hp 502's in the '90 and up 292's they are around 70-71 mph. My engines are putting out all of 415 hp and I cannot get anywhere near those speeds. The only thing I can think of is the drives are at their limit. The boat also feels terrible at WTO, completely not responsive. I've spent hours messing with the the tie bar with little to no improvement.

Endeavour, That does seem high. Does the 292 have the built in hook near the chine at the transom? The 311 doesn't, the others do. Here are my numbers at wide open for reference since my boats are Formulas:

311 23 mirage plus: 8%
272 25 mirage non plus: 11%
206 (single) 23 mirage plus: 16%

Friends 242 23 mirage plus: 13%

how far the drives are down in the water makes a big difference as well...not sure what X dim they used then...

Mine does have hook right in front of the tabs. Prior to this I had a 242 running a Mirage plus 21 and my slip numbers were similar to your friends. Mine was 12.5%. At 69 mph in the 242 so little of the boat was in the water that the hooks didn't touch the water. Now the 292, is much heavier and the hooks are definitely in the water, but I suspect the issue is in the drives or the props.

I've been debating getting new cases or putting nose cones on my old drives. However i have been thinking about adding nose cones for years and it seems that every one that adds them says they are worthless. I don't want to start pi$$ing money away. So I am really leaning towards the newer style cases.

I'm really not sure what the x dimension is on the 292. I know that they are deep. I have measured it before but it was a long time ago and I have now forgot what it was. If I have time today, I'm going to give BBlades a call and see what his take is on this.

Ran one of these over 140 seemed to work fine, may be cheaper than new cases, http://imcomarine.com/cal_store/driv...nose-cone.html I would be looking at the hook in the hull.

I was looking at that. I have zero experience with nose cones. From what I read on hear it seem that the imco cone like to be near the surface, my drives are deep. For $107 it's worth a try. How deep were your drives and what kind of boat did you run it on?

Stock X , standard length drive Mach 26 DCB, I tried an Imco lower it didn't change a thing speed wise.

This is my first boat with twins, so a lot of this is new to me. Here is another thing that happens about 63 or so and get worse with more speed. When trimmed out and WTO I need to slowly turn the steering wheel to keep the boat going straight. It can be as much as 1/4 to 1/3 of a turn. When I pull back on the throttles its a white knuckle experience, it feels as if it could hook up and dart, of course it doesn't but it is not a pleasant experience. I've always thought this was a clear sign that I needed external steering; however, the more I read posts it sounds like my drives may be blowing out. Does anyone have any input on this?

Just curious, what was the design speed of the hull ?

No idea- but I know it isn't 67 mph. There are guys with 90-91's with 502's and the modern bravo lowers and they are in the 70's. Mopower has a 272 that does over 80 mph. I'm no where near the hull speed of the boat. There is an issue somewhere, and the more I dig, the more I think its the lowers. I just don't want to spend the money replacing them only to find out it was another issue.

Good luck

I want to fix the problem. I'm sure its something simple,however knowing what to fix is the issue.

At that speed I don't think it is the lowers. My 272 is rock solid at 65. Is there any play in the gimbals? Or cable helm? The gimbals on the 272 were worn and I did not like it, i rebuilt them and good to go.

I had this exact same condition (backing off the throttles) turns out it was a bad steering ram. Every time i'd get big air and land the boat would get squirrely too. I went full hydraulic. Problem solved.

As far as slip - your tach's might not be accurate. The tach in my 1989 Mirage reads a few hundred high.

Talk to Tom at Hydromotive Props. I don't know if they still make it but they used to have a nosecone for the older Bravo One. They were designed to prevent blow out on the old style drives. Eventually Mercruiser changed the design and it wasn't needed.

One thing I was thinking about last night is that Formula put a large mushroom thru-hull under the starboard engine. This is for the overboard discharge of the head. I've always wondered why they put it there. To me at speed, it would seem that it would act as a brake. Not only slowing me down but also making the hull surface asymmetrical. I'm curious if this could be causing the props to slip a bit and to be causing the starboard pull at higher speeds?

Sailboat racers always remove mushroom thru-hulls and replace them with flush thru-hull and then fair them in so they are perfectly smooth. If they are worried about the drag at 6-8 mph, then what are they doing at 60-70 mph? I think this is at the issue!