From Old to New; Any Effect On Motors?

[Mr. Osprey]

If I were to repwer my 170 with say a new 75hp engine, would I see an increase in top end form the approximately 29MPH I already get? When I finally do repower, is there any way to reach 35ish (or at least break the 30 mark) with a 75-90hp motor (with the use of setbacks, doelfins, hydrofoils,weight distribution)? What do ya'll think?

[JimCt]

You should have no problem breaking 32-33 mph with a 75.  I have a 2003 Merc 75 2 stroke on my 170 and I can eek out 33+ with 2 people aboard and 25g of fuel.   I maybe slightly under-wheeled because my tach shows me at the high end of the rev. window.  Don't know what effect a foil will have but I will be putting a Bob's Machine one on this spring so we'll see.  Rickk and a few others have had foil experience.  Maybe they can help you out on that question.  But I'd guess on a flat day with a reasonably light load you may make 35.

[RickK]

M.O. - what year motor did you have?  Old one's HP were measured at the head and the new ones are measured at the prop. When they made the change, I don't know.  That being said, I went from a '88 110 evinrude that would push my boat at 38mph (gps) to a '98 newly rebuilt 115 johnson and it pushes the boat at 39.9 mph (gps).  I added the Bob's stabilizer plate and I'm back down to 38.4mph all trimmed out and tweaked.
I think Miquel gets about the same with his 115.
Jim, you get 32-33 on GPS from a 75hp? That's a strong motor.

[Mr. Osprey]

It's a V-4 1984 Mercury. That's the other thing. How come my 22 year old Merc is built with 4 cylinders while all the new engines at the same horsepower are V-3's? Does newer technology mean less cylinders to get to a certain HP? JimCT, you have a 1970's 170, correct? I think you might be getting 32-33 because your boat is lighter than might 80's 170. Is this possible?

[JimCt]

M.O.,

Lots of variables will affect your top end speed.  Major factors of course are weight and HP.  I'd be suprised if there is any signifigant weight difference between our two bare boats.  I keep my weight loading down to absolute minimums.  The Merc. has maybe 80 hours on it so it is likely performing as new.  I carry a 6hp '72 trolling motor which adds 60 pounds or so. Also have a 3g portable reserve fuel tank stuffed up in the bow locker.  If I pulled that off and had only 10g of fuel aboard and only me aboard (150 lbs.) and a little higher pitch prop, my guess is I could squeeze out another mile per hour or so.  Conditions would have to be glass calm, a shaved head and no change in my pocket...

Concerning the motor, if the cylinder compression is not up to spec. you will not develop the advertised HP. Carb jets & float levels have to be right too.  Funny thing about my 75 Merc.  Weight, stroke, displacement is the same as the 90 hp.  Only difference is the carbs.  If I wanted more speed, which I don't, I could up the engine to 90 simply by changing carbs.

[Deck Daddy]

I get 33 on the gps with a 1991 90hp yammy and I have extra drag with the stern box's. Thats with 2 people and gear. Fast enough for me and great fuel consimption.

[Mr. Osprey]

After looking through the advertisements, I was able to find out that the 170's of the 70's era weighed about 900 pounds while the 80's hull weighs about 1150. That's a 200-250 pound (give or take on what's in the hull) between the two boats. Does 200 pounds effect the hull's top end that much (as to just the hull and no additional equipment)?

Rick, why does your hydrofoil take off form your top end speed? Does it have to do with friction in the water?

[JimCt]

200 lbs. will cut speed. How much would have to be tested.  On these 170's every pound counts.

 Have to say that 30+ mph is plenty fast for me.  Only open to WOT usually once per trip to blow out the carbon.  Foil may cut speed a tad because of its slight extra wetted surface.  I got the foil to help stabilize the mild porpoising I have at around 25-28 mph.  Worthwhile trade-off for me.

[RickK]

After looking through the advertisements, I was able to find out that the 170's of the 70's era weighed about 900 pounds while the 80's hull weighs about 1150. That's a 200-250 pound (give or take on what's in the hull) between the two boats. Does 200 pounds effect the hull's top end that much (as to just the hull and no additional equipment)?

I'd have thought the newer were lighter, with newer technology glass, resin and less of each, trying to eek out more profits.  Still learning after all these years.

Rick, why does your hydrofoil take off form your top end speed? Does it have to do with friction in the water?

Beats me - what Jim says sounds good.

Have to say that 30+ mph is plenty fast for me. Only open to WOT usually once per trip to blow out the carbon.

I usually cruise at 4000(30mph) - 4500rpm which is about 33mph (5300ish rpm is max), and of course have to "blow out the carbon" every now and then.

I get 33 on the gps with a 1991 90hp yammy and I have extra drag with the stern box's. Thats with 2 people and gear. Fast enough for me and great fuel consimption.

A 90 and a 115 are not that different in fact I read that you can change the carbs on a 90 to make a 115.  The engine changes when you get above a 115.

I got the foil to help stabilize the mild porpoising I have at around 25-28 mph.

Jim, thats odd that you get that porpoising.  At that speed, which I assume is your cruising speed, do you have her trimmed up max?  I only get porpoising when I over-trim.  I didn't need the plate for that, I got it to help with the holeshot and to raise the backend a bit while on plane.  Not sure it helped either.  I may buy a Doelfin yet and test the difference between them - not sure I want to drill more holes in my cav plate though.

[Mr. Osprey]

Is a 70-75hp engine OK for watersports? My father tells me that it's bad for smaller engines (compared to say a 90 or 115)? Part of my concern is not only the top end but the torque, strength the motor has to pull an adult skier or wakeboarder. If I had the oppurtunity to buy a strong 75hp engine that has lots of power but only would get me to 30mph or a little over that, I would be fine. Does such an engine exist?

[Radioshop]

My philosophy on engines is this; you don't have to go full throttle, but it's hard to add extra ponies at sea when you want them ie, when you're out running weather etc.  Get something a little bigger after all a bigger engine that doesn't work so hard lasts longer than a smaller engine firewalled most of it's life.

[Anonymous]

Interesting discussion. I h ave owned a 1971 for many years. It had a 70hp engine and always seemed under powered. I recently put a 120hp  and the main reason was my son, a marine, wanted to go skiing. The first thing that happened was very bad porpousing.  I installed manual adjusting trim tabs and I am am pretty sure that I am running close to 50mph. I will have it checked. Pat