Dumb speed question

[fabrimacator21]

How much mph should I look to gain when going from full trim down to full trim up(or atleast trimmed for top speed)?

The boat is in my avatar. Motor is a 200hp 305 chevy and it has the jackscrew power trim.

When my trim worked, my speedo didn't and neither did my tach. Now I have a speedo and tach but my trim doesn't and it's stuck as far down as it can go, meaning the nose of the boat is buried.

I got 44mph at about 4100-4200rpm. What kind of rpm and speed should I see with it trimmed out for top speed? I ask because I'm on the fence for buying a 19 pitch prop. I'm running an old chewed up 17 but I'm thinking about going with a 19 pitch turning point vented prop.

Is the rule 100 rpm lost per pitch (when going to a higher pitch prop) or 200 rpm lost per pitch?

 

[Snobike Mike]

Re: Dumb speed question

In many cases the difference between trimmed down and trimmed up can be 5 mph or possibly even more depending on the hull/boat type.

Regarding the rpm changes, it really depends on the pitch but also if its cupped/non-cupped, etc.

Cheers.

 

[blouderback]

Re: Dumb speed question

I agree with Mike. I generally see about 4-5mph increase when I go from full down to trimmed properly.

 

[Philster]

Re: Dumb speed question

On an all-out, deep V, performance planing hull, you could easily gain 10-15 MPH.

On My Scarab, if I never touch the trim, I am stuck lugging the engine at 48 MPH. If I trim the outdrive up to the sweet spot, I am over 60 and climbing. The best high-speed prop might allow a touch more up trim, or more bite alone at a given trim height, and another 1-2 MPH.

It has a ton to do with power and hull design. Props matter, too, but they are bit players in the act.

 

[Silvertip]

Re: Dumb speed question

Why are you considering a prop change when the trim doesn't work? The rpm gain from a pitch change is a crap shoot since there are too many factors at work. But the "rule of thumb" (not the absolute) is IF you stay with the same prop design series (same make and model from the same manufacturer except for pitch) THEN you may see a 150 - 200 rpm change up or down from a one inch decrease or increase in pitch. If you switch from a 19P Merc prop to a 19 Michigan they may perform very differently. Get the idea? Now if you not only change manufacturers, but you change pitch, change from aluminum to stainless, rake, number of blades, etc you could see drastic changes one way or the other and quite possibly NO change whatsoever.

 

[outdrsmn82]

Re: Dumb speed question

i was thinking the same thing. fix the trim & then move on to changing props IMO.

 

[QC]

Re: Dumb speed question

Yup, fix trim first, verify tach is correct and use GPS speed. Every number has to be dead on and your RPM will increase dramatically along with speed when you trim up. So everything you are trying to accomplish starts with WOT RPM at max speed (best trim) and average load (people, beer, stuff). There is no way to make decisions with out all of that, and you, err, well, uhh, don't have that . . . Good luck

 

[steelespike]

Re: Dumb speed question

As all have suggested you can't make an accurate prop change without
a base line best rpm and speed.It would be a very good idea to get the old prop fixed(check price) and get that base line performance.If a prop change is indicated keep old for a spare.I wouldn't be surprised to see your present setup over rev at best trim.Do you know the max rated rpm for your motor?Pretty much impossible to tell much about the boat from a little pic.could be 20 ft or 15 ft.Also would be good to know the details on the drive.

 

[QC]

Re: Dumb speed question

I am guessing OMC stringer as he mentioned "jack screw" trim . . . However . . . I thought the jack screws were only on the 2.5s and 3.0s . . . V8s had two hydraulic front motor mounts if I recall correctly.

 

[fabrimacator21]

Re: Dumb speed question

I agree,,, I need to get the trim working before I buy a prop. I'm more just trying to guestimate what my top speed would be with a good prop and trimmed up. Also I wanted to know if my rpms were ballpark since I recently rebuilt the carb.

Rpm range is 4200-4600 in the book IIRC.

It is a Omc stringer drive and it does have a jackscrew with a "bridge" type motor mount(goes from one stringer to the other) up front.

I'm guessing I could squeeze upper forties, maybe even 50mph out of it with a good prop.

The current prop is just the original OMC 17P prop from 1979. It has a very slight cup to it.

Oh and it is a 16ft(18ft if you count the I/O).

 

[Philster]

Re: Dumb speed question

if your engine has good torque and HP at the top end, you might get 'er up there, but I am gonna bet you are stuck in the 40's no matter what.

 

[Silvertip]

Re: Dumb speed question

There is a difference between what the book says for wide open throttle rpm and what you "actually are getting". If the engine runs at or near the upper end of the manufacturers recommended rpm band (4200 - 4600) with what you consider an average load, then you already have the best prop. If not, then you need to change pitch up or down to get the engine in that band. Top speed will be whatever that prop provides. You cannot shoot for a top speed if the engine cannot spin a prop that provides that speed.

 

[fabrimacator21]

Re: Dumb speed question

what kind of rpm gain are you guys see when going from trimmed down to trimmed up?

 

[QC]

Re: Dumb speed question

As much as 400 RPM for me.

 

[Texasmark]

Re: Dumb speed question

How much mph should I look to gain when going from full trim down to full trim up(or atleast trimmed for top speed)?

but I'm thinking about going with a 19 pitch turning point vented prop.

Just commenting on this comment. I only had one vented prop. It was a Merc brand prop, 3 blade and it was on a deep V pad Ranger that was heavy.

On hole shot or when pulling skiers the performance of the 3 holes was outstanding. It was like having 2 props in one. Low end torque was immense.

What was cool, was that once you got hooked up and were moving on across 35mph or so, there would be enough water flowing across the outside of the prop to seal the holes and the prop would only get water across the blades rather than exhaust gasses and water. At that moment, the engine would down shift with the added load, almost like you shifted gears in a car and the boat would take off. I just loved it.

Mark

 

[fabrimacator21]

Re: Dumb speed question

Cool

Although I'm starting to rethink going to a 19.... took it out this weekend with the chewed up omc 17 and it didn't exactly shoot up on plane with 3 people gear and a full cooler.... all the weight was in the back but still, I think a 19 would be too much prop for it.

It's looking like a vented 17..... I'll have to live with 45mph.

 

[Texasmark]

Re: Dumb speed question

Cool

Although I'm starting to rethink going to a 19.... took it out this weekend with the chewed up omc 17 and it didn't exactly shoot up on plane with 3 people gear and a full cooler.... all the weight was in the back but still, I think a 19 would be too much prop for it.

It's looking like a vented 17..... I'll have to live with 45mph.

Lookup DHadley, or Walleyehed on here. They know of the exact brand/model of props which give you bow lift or stern lift or whatever you want; all has to do with how the blades are formed. Course I personally would go with the holes regardless, unless the prop they recommended didn't have them. Then I would ask them and (www) the mfgr. why no holes?

They will give you the answers you need.

Mark