Up one hole, Reaching RPM range

[TheOtherLine]

I finally got a chance to get the boat in the water since having it hauled out to do the 200 hr maintenance, wax it, add the kicker, etc.,

Anyway, while doing the maintenance, I had the motor raised by one hole and here is what I've noticed.

1) Used to have to really tweak the motor trim to get the rpm's up. I would rarely get past 5800. NOW, gets to 6000 rpm no problem.

2) At 4500, speed increased by about 2 mph. Fuel consumption seemed to stay the same at 2.2 mpg.

So far, I think raising the motor really helped. I will take more accurate data when I can get out during the week and not have to dodge all the weekend warriors.

 

[Brian S]

Which hole do you have it mounted on now?

 

[TheOtherLine]

Which hole do you have it mounted on now?

2nd hole up from the bottom hole.

 

[TheOtherLine]

Don't stop now. I know of nothing to gain, dragging a lower unit too deep.

The benefit of having a lot of prop in the water in large, following seas :?:

 

[TheOtherLine]

OK, went out and ran the boat through some trials. I know it's not a speedboat and speed shouldn't matter but I just like to know what the boat will do.

After comparing the new numbers with the old numbers, I didn't see that much of a difference. The primary difference was a noticeable difference in my ability to reach higher rpms at all trim/tilt levels.

I wish some of you better informed people would tell me why raising a motor 1" could make such a difference. Can't be that much more drag, can there?

Interestingly enough, unless my memory is playing tricks on me, I recall seeing more 2.3 - 2.4 mpg #'s in that 4200 - 5200 range that I saw today. :?

To really con volute things, the new numbers are not entirely from raising the motor one hole. I did have the prop off and tuned, so I'm not sure that these numbers have any value at all.

Sure was fun racing up and down the ICW though :lol: :lol:

 

[grouperjim]

ah....grasshopper, experimentation is a good thing. all things are interrelated. the temp of the water, the tuning of the prop, the friction of the LU in the water due to engine height, the A/V plate and it interaction w/water etc. etc.

while raising the motor to reduce LU friction with the water, you also reduce the effectiveness of the A/V plate as a control surface to raise the bow when the motor is trimmed up. raising bow reduces wetted surface of the hull/friction at the waterline.

so, by raising the motor you reduce friction in one area and possibly increase it in another. it's all good. check it out, and if you don't like the new handling characteristics, you can always put it back the way it was.

 

[TheOtherLine]

"Can't be that much drag"

yep, don't stop now. :wink:

I have no idea what you are saying. :?

 

[Catfish]

TheOtherLine

What prop are you running now during these latest trials, make and model? What results have you had with 4 blade over 3 blade props.

Catfish

 

[TheOtherLine]

TheOtherLine

What prop are you running now during these latest trials, make and model? What results have you had with 4 blade over 3 blade props.

Catfish

Smooth Conditions.
Little over 1/2 tank of fuel.
2 people.

17 pitch Yamaha black stainless
4000rpm 20mph 2.1mpg
4500rpm 25mph
5000rpm 30mph
5900rpm 40mph

17 pitch Mirage Plus
4000rpm 16mph
4500rpm 27mph
5000rpm 34mph 2.2mpg
6000rpm 40.5mph

17 pitch Revolution 4
4000rpm 22mph 2.0mpg
4500rpm 30mph 2.2mpg
5000rpm 35mph 2.3mpg
5800rpm 40mph

17 pitch Powertech OFS
4000rpm 20mph 1.9mpg
4500rpm 27.5mph 2.2mpg
5000rpm 35mph 2.3mpg
5900rpm 38mph


I am running the 17p Rev 4 and like it.

 

[TheOtherLine]

The higher yer raise lower unit, better boat runs, as long as prop has bite.

Aahhhh. OK. So that's what you meant by, "don't stop now"....

Although my technically meticulous compadre seems to disagree with that assesment. It's not that easy. If it were, everyone would just raise their motors. I think there IS a balance between raising the motor and the effectiveness of the AV plate and maybe even some other things...

I haven't had enough time to play with it so right now I am not a come to any conclusions about whether or not it was a good idea. I guess the next time I am in a serious following sea, I'll be able to compare the boat's performance to when the engine was lower.

 

[TheOtherLine]

Interesting read. Would seem to confirm what BOTH of you have said.

So, as my motor is as far aft as I can get it (on a bracket ), raising the motor reduces drag (therefore more possible speed), but also reduces effectiveness of motor trim ( in terms of raising / lowering the bow ).

Which is where my trim tabs come into play. However, trim tabs only help if the boat is running at a fairly decent clip. In other words, adding Tab isn't going to do much for a boat running at 10 knots in snotty seas.

Right or Wrong?

 

[TheOtherLine]

In other words, adding Tab isn't going to do much for a boat running at 15 knots in snotty following seas.

Yer wanna run bow down ("adding Tab") in a following sea?

OK bad example, the point is, applying tabs aren't going to do anything for you when running at slower speeds.....

Jeez...

 

[Catfish]

TheOtherLine

Aside from all the back n forth jargin, under normal conditions how does your boat handle with the Rev4 prop? I have been thinking about buying one and would appreciate your experience with it.

Catfish

 

[MaxOut]

Just get a jack plate and adjust it on the fly :lol: It's only money and getting the motor back will help as well :lol:

 

[TheOtherLine]

TheOtherLine

Aside from all the back n forth jargin, under normal conditions how does your boat handle with the Rev4 prop? I have been thinking about buying one and would appreciate your experience with it.

Catfish

I like it a lot. The word that always seems to come up when describe this prop is "bite". The prop just seems to dig in and provide me with that stern lift that allows me to get up and stay up on plane, at lower rpms.

I may have given up some fuel efficiency but I hated the 3 blade and didn't have it on their long enough to run jargon...