Big Repower

Hi I have a 1967 silverline 18

Hi I have a 1967 silverline 18 foot kingfish closed bow I have fully restored. It currently has the original merc 110 and pre alpha o/d I have had both fully rebuilt and restored. All in all I am very happy with the performance however I would like to go faster at top end. I curently get to nearly 25 mph on a calm day but I would very much like to get near 40. My question is what would be a good engine o/d combo to upgrade too and how dificult might it be to do the job?

"25 mph seems mighty slow for

"25 mph seems mighty slow for an 18 footer with 110 hp. Are you sure your speedo is correct? Is that boat really heavy?

We all want to go faster, but bear in mind that for boats the increase in speed will be the square root of the increase in power.

That means that if you double your propshaft hp, you will only increase your speed by 41%.
In your case to hit 40 mph, you will need about 250 hp. That seems like a huge amount of power for an 18 footer, and that thing should fly. Again it seems like something is wrong that you can't hit more than 25 mph now.

Rod"

"It is alway good to check you

"It is alway good to check your measuring technique against a known reference, especially a boat speedo.

If your waterway has mile markers simply time yourself against the "known" distance. Or use a hand held gps.

Post a picture of your restoration, it sounds nice.
Mark"

Use a GPS to check your speed.

Use a GPS to check your speed. What does the boat weigh? Did you remove floor and remove the wet floatation foam when you restored it?

a 67 Silverline closed bow is

a 67 Silverline closed bow is probably a tank but I bet it rides nice! The least expensive thing to do is play with different prop set ups. Have you tried that yet? 110 hp isn't a lot for a boat that size but you should be able to do at least 30 if it's propped right. My 170 hp in a boat maybe the same weight as yours does 42 WOT with my 4 blade SS prop. I only got 38 with the stock mercruiser black alum that it came with. The hole shot seems about the same amount of time to get up on plane using the trim tabs. It's a place to start and a lot less cash than a whole repower and outdrive!

Thanks for all the information

Thanks for all the information. I spoke with my mechanic who told me he tuned my engine down for trolling plus switched out my prop for more low end power less speed because I told him I wanted a good fishing boat. The cavatation material and floor is new I did use ¾ inch plywood for the floor which adds a great amount of weight plus very heavy captain’s chairs that are easily over 80 pounds each I had gotten from a junkyard because they matched the boat very well plus I am a 300 pound man. All together im sure I am carrying a lot more weight than I actually need lol. As far as the repower my mechanic said I should keep my o/d and go to a Chevy 4.3. However I am going to try another prop and possibly buy trim tabs since the only method of trim I have is a hydrofoil mounted to my o/d and manual trim position. I will let yall know how it goes when Lake Erie thaws and I can try it out.

"hmm all the times i was on la

"hmm all the times i was on lake erie it wasn't falt enough to go over 20 good jetski lake though "