Jumped 700 RPM

fishhawk16 · Jun 12, 2006

I have 1996 90HP Looper on a Crestliner Fishhawk 165 SC. Dry weight is about 900LBS. The ventalation plate sits level with bottom of the boat. When I got my boat it had an OMC 14x21 alum prop. Boat would turn around 4800rpm. Knowing that this was to low I had a 14x19 repaired (minor ding) and put that on the boat. RPM was now 5100rpm. Still not enough I purchased a 14x17 OMC alum. I now turn around 5200 wot with a speed of about 43mph (speedo not GPS). I then tried a brand new 13.25x15 Bombardier. This one had a different blade design than all the rest. With this prop I was turning 5900RPM wot with a speed of 42mph. All speeds are with the speedo - same load same day. Motor fully trimed up All props are OMC or Bombardier All props are alum Questions What is the max RPM range I can run the engine at with out damage? Any ideas as to why I jumped 700RPM with only a 2" pitch change? If I put on a 13.25x15 SS will my RPM change up or down significantly? Sorry for the long post

Dhadley · Jun 12, 2006

Re: Jumped 700 RPM

The big difference is you changed blade design. Shape and diameter. The last thing you want to do is change to a lower pitch to gain rpm. I have a hard time thinking you'd have to go below a 19" prop on that light of a boat to get the rpm's you need unless it has a ton (literally) of weight in it.

fishhawk16 · Jun 12, 2006

Re: Jumped 700 RPM

I'm with you Dhadley. I think the weight is much higher. I really cant say for sure but based on the way it tows and how it feels in the water this is no ligh weight boat. I'm using my old 14.5" Thundercraft (FG) as a reference. So the blade design would make that big of a difference? And 5900RPM is fine?

ufm82 · Jun 12, 2006

Re: Jumped 700 RPM

I can't help you on the WOT rpm question but the prop question is easy. Blade design, surface area, rake, cup, etc all have effects on the RPM's your engine can turn. Some props bite better than others and some bite less. I can run a 19 prop (Merc prop) in a Laser, a Mirage, a Tempest or a Trophy and get 4 different rpm readings. Blade design has a huge effect on engine RPM. Changing from an aluminum prop to a stainless prop all by itself will change your RPM. You should raise your engine until the anti-vent plate is about 1 1/2" above your keel. This will lower drag and increase your WOT. As D stated, a 19 pitch is probably the lowest you should need to go on that ride- a 90 should push that boat very well. Going with the 15 pitch stainless will definitely overdo the RPMs. I would try a 13.25 X 19 if anything. Dhadley- what do you mean by the comment about dropping pitch the raise RPM? That's normally what most people will do first when setting up a boat. Can you explain? UFM82

fishhawk16 · Jun 12, 2006

Re: Jumped 700 RPM

Thanks for the replies, The boat fully loaded is likely closer to 2000lbs, dry weight was boat only, no motor gas etc. I'm getting more and more confused here. I just wrote all this down on one

peice of paper. The 14x21 turns 4800 The 14x19 turns 5100 The 14x17 turns 5200 (mabe something wrong with this one but bugs the heck out of me, cause other than some chips out of the diffuser ring its in great shape - no rework) The 13.25x15 turns 5900 In summary, with the expection of the 14x17 I'm jumping 300 to 400RPM per 2" of pitch. What do you think?

rickdb1boat · Jun 12, 2006

Re: Jumped 700 RPM

Pretty light boat: (Dry Weight/ No motor) Length: 16' Model Name/Description: 1650 SC FISH HAWK Boat Type: Outboard Boats Hull Material: Aluminum Beam: 7' 1" Engine: Net Weight: 870 I think a new 17" is gonna be the best...

Dhadley · Jun 12, 2006

Re: Jumped 700 RPM

5900 is fine for that motor. If it were me I'd try to work with set up and the larger diameter 19. Or at least the 17. Raise the motor to reduce drag and gain rpm. That will also increase overall efficiency. Naturally these being aluminum props we might not get them to be as efficient as a ss prop but we can try. UFM82, we always try to work the set up to gain rpm with an outboard. A sterndrive is harder because it's a lot of work to raise the outdrive and motor. So we're kinda stuck with changing props. With an outboard it's fairly easy to change the X dimension by raising the motor.

fishhawk16 · Jun 12, 2006

Re: Jumped 700 RPM

Ya that looks about right, mine is a 1997 but about the same dry weight. So take 900Lbs of boat add fuel 160lbs add motor and oil 350lbs add batteries add a couple of guys and gear 500lbs and you got around a ton. With this information does the prop information sound about right? Thanks

fishhawk16 · Jun 12, 2006

Re: Jumped 700 RPM

Thanks DHadley Last summer, based on some recommendation that you and some other good folk here shared I started playing with the motor height. I rased it 2 holes (think I have one to go). It improved the overall feel of the boat for the better. Steering was neutral when trimed correclty. Before that it would alway pull to one side (port) - can't remember the RPM change. Either way the one hole will only make about 150rpm difference - right? Do you think that I could get the RPM's up that much (500-600) with setup? Other than the motor height. I'm not sure what to do? Any recommendations? Sorry - I should had mentioned this earlier I completly forgot. Last year I had some stuttering under heavy load acceleration. A couple of weeks ago when I put the 13.75x15 prop on it came back, and this was under normal load. I put the 14x17 on and it went away. When I came home I drained the carbs and found some water in the fuel. I then drained the entire fuel system in the motor and about 4 qts of gas out of the tank until there was no visable signs of water. I then added some OMC fuel treatment. Other then when it was under acceleration the boat ran great even with the water in the fuel system. Could this be having any type of affect on my numbers?

fishhawk16 · Jun 14, 2006

Re: Jumped 700 RPM

OK I raised the motor one more hole this morning (bottom hole). Only took about 45 min. This brings the anti vent plate about 1 1/2" above the bottom of the boat. I'm hoping to get it in the river tonight. I will try the 17 and possibly the 19" and report back in 24hrs.

fishhawk16 · Jun 15, 2006

Re: Jumped 700 RPM

Before I went to the river last night I picked up another prop from the dealer it was an OMC 13.25x17". My thinking was that since my RPM jumped so much (700RPM) going from a 14x17 to a 13.25x15 I thought that the new design OMC 13.25x17 would put me right where I needed to be. I started out with the 14x17 since that was already on the boat. I saw 44-45Mph at 5300. The RPM was what I expected but my speed jumped 2mph (not GPS). So it looks like I picked up a little RPM with the motor raise. I then put on the 13.25x17 the results were disappointing. I saw 44-45 @ 5300, no change in RPM. I expected to see at least some RPM improvement. I didn't have the 15" any more so I couldn't try that. I can still fully trim it up with out blowing out, although its very close; as soon as I hit any waves (remember this is a river) the RPM climb quickly and it start to porposing. I then need to trim down. I do pick up about 100-150 RPM when I trim up. Not sure what else to do other than order a 15" SS. The motor is in the bottom hole and as high as I can get it.

Dhadley · Jun 15, 2006

Re: Jumped 700 RPM

How about a manual jackplate?

fishhawk16 · Jun 15, 2006

Re: Jumped 700 RPM

That would alow me to get the motor up? What else would it do? and How much does something like that run?

Jumped 700 RPM

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