motor placement

[sadam68]

i have seen some posts refer to the height of the motor on the transom. what does this effect and where should the motor be set? i have an 18 thompson with a 70 hp johnson. i would like to get a new prop next season as this one is a 13 1/4 x 17 and the boat is sluggish with more than 3 people in the boat(no tach yet). i would like to make changes to get a little more out of the motor in the spring but would also like to make sure everything is correct before buying a new prop. is this something that would be found in the manual or is it trial and error? any info would be great, thanks, scott

 

[Stumpknocker]

Re: motor placement

I'm sure you will be told to first get a tach. Then go from there. Thats what I was told and once I knew what RPMs I was actually running I was able to get the right prop. I ordered a Tiny Tach for around $35 and it seems to work fine. Digital with a 5 year battery.

 

[Scaaty]

Re: motor placement

a 18 foot Thompson with 70hp and 3 people will not improve with raising the motor a touch...that only applies to race boats to get less drag (I had a surfacing two blade on one of my old boats). You need to upgrade boat and motor for more zip.

 

[walleyehed]

Re: motor placement

Lets look into this a little....<br />I've increased speed by as much as 7MPH on an 18ft Starcraft just by lifting engine 2 holes and adding a simple 5-1/2 inch aluminum channel spacer (set-back), and had to raise pitch due to RPM gain and then gained another 1 or 2 MPH.<br />Costs ya nothing to raise the engine on the transom and try it. Loosen bottom 2 bolts, remove top 2 (have engine supported so it can't fall) Move it up one hole at a time and test...see how far up you can go before the prop wants to ventilate (loose bite) you may gain 100-150 RPM per hole.<br />70hp on an 18 is a little shy possibly, but you may be able to help it some.<br />As gil009 said, a tach is the best instrument you can have, but I don't think you are over-reving with the set-up you have.