Does prop slip increase or decrease with speed? Top speed less than expected.
10-28-2014, 07:31 PM
#1
Does prop slip increase or decrease with speed? Top speed less than expected.
I ran my boat Sunday for what I was planning to be the final run for the season and finally had a chance to really crack on it before it went on the trailer. Seems like I always had a crapload of people on board every other time I had it out this year, so I went easy on it for most of the season. Top speed was considerably less than anticipated. I plugged in some numbers into the go-fast.com prop slip calculator, and I got the following slip calculations:
I am running a 1.36 Bravo 1 with a Mirage Plus 23. Boat is a 1988 Baja Force235 with a 489 running AFR heads, Holley 800, Lightnings, etc. This build has been pretty well documented on other threads here, so most of us are familiar with it. I had the engine dynoed and saw 585 HP at 5400 RPM and between 575-600 ft lbs of torque all the way from 3000 RPM up to peak HP.
At 3000 RPM I was seeing about 39 MPH on GPS. Theoretical MPH at this RPM is 49, so I have a slip factor of around 19% at this speed. Fair enough, there is a lot of hull in the water at this speed.
At 3500 RPM speed was showing about 48 MPH on the GPS. That equates to a slip factor of 14%, which is pretty much as expected.
I ran it on up to WOT, and was only able to make it to 61 MPH. RPM was only 4600, and that comes out to a slip factor of 18%. Something seems amiss here, both from the higher slip number and the fact that I wasn't able to spin this thing faster. I played around with trim and tabs, but did not seem to make a difference on top speed. Why would my slip number go up, unless I had it over trimmed?
It seems to be making plenty of power at lower speeds, getting up to 50 MPH is pretty effortless, so engine seems to be running OK. Did not note any miss or notice that it seemed to be running poorly, but it just wasn't bringing the power. Dyno runs were made with dry dyno headers, and A/F numbers were around 13 from 4000-5000 RPM, so I would expect it to run a little richer with the wet exhaust and muffler tips on it. I have yet to check the easy stuff like throttle opening all the way (it was with the other engine, and nothing changed in this regard), timing, etc. I was running speeds like this with a 454 that made less than 400 HP, so I would have to be dropping at least 100 HP if it is an issue with the engine.
Anyone want to take a stab at it? Maybe my boat is just a pig!
I am running a 1.36 Bravo 1 with a Mirage Plus 23. Boat is a 1988 Baja Force235 with a 489 running AFR heads, Holley 800, Lightnings, etc. This build has been pretty well documented on other threads here, so most of us are familiar with it. I had the engine dynoed and saw 585 HP at 5400 RPM and between 575-600 ft lbs of torque all the way from 3000 RPM up to peak HP.
At 3000 RPM I was seeing about 39 MPH on GPS. Theoretical MPH at this RPM is 49, so I have a slip factor of around 19% at this speed. Fair enough, there is a lot of hull in the water at this speed.
At 3500 RPM speed was showing about 48 MPH on the GPS. That equates to a slip factor of 14%, which is pretty much as expected.
I ran it on up to WOT, and was only able to make it to 61 MPH. RPM was only 4600, and that comes out to a slip factor of 18%. Something seems amiss here, both from the higher slip number and the fact that I wasn't able to spin this thing faster. I played around with trim and tabs, but did not seem to make a difference on top speed. Why would my slip number go up, unless I had it over trimmed?
It seems to be making plenty of power at lower speeds, getting up to 50 MPH is pretty effortless, so engine seems to be running OK. Did not note any miss or notice that it seemed to be running poorly, but it just wasn't bringing the power. Dyno runs were made with dry dyno headers, and A/F numbers were around 13 from 4000-5000 RPM, so I would expect it to run a little richer with the wet exhaust and muffler tips on it. I have yet to check the easy stuff like throttle opening all the way (it was with the other engine, and nothing changed in this regard), timing, etc. I was running speeds like this with a 454 that made less than 400 HP, so I would have to be dropping at least 100 HP if it is an issue with the engine.
Anyone want to take a stab at it? Maybe my boat is just a pig!
10-28-2014, 07:45 PM
#2
Registered
iTrader: (2)
Bud are you sure you are running on all eight holes? With almost 600hp I would think that little mirage p-shooter would be annihilated especially with that kind of slip. I would pull the plugs and see what they look like. I've said this b4 but been argued the 3blade is not the way to go. Make sure you don't have a run issue then worry about the slip.
10-28-2014, 07:53 PM
#4
Bud are you sure you are running on all eight holes? With almost 600hp I would think that little mirage p-shooter would be annihilated especially with that kind of slip. I would pull the plugs and see what they look like. I've said this b4 but been argued the 3blade is not the way to go. Make sure you don't have a run issue then worry about the slip.
Buddy of mine has virtually the same boat (89 Baja Force) and tried a 4-blade Bravo 1 Prop and it did not like it. Geve him too much stern lift, when what these hulls really seem to need is bow lift. The Force models have the helm pushed up further than most go-fast hulls, so there is probably more weight centered towards the bow. Mine seems to run with the bow pretty well planted most of the time.
10-28-2014, 07:56 PM
#5
Maybe the dyno was a little "happy" - no real way to know for sure. The power numbers were in line with what Bob M expected with the cam and AFR heads that are being run - they weren't out of line for similar builds. Even if I'm not making the advertised power, it would have to be "happy" to the tune of at least 150 HP for these speeds.
10-28-2014, 08:14 PM
#8
You should be able to put the fat lady in the cabin of that boat and still scream along pretty good if you truly have over 575 ft lbs of torque from 3000-5400 rpms. How is the bottom of the boat ....Does the boat seem to free up at speed ..Or plough water ...Something doesn't add up ...Who dynoed engine..........?
10-28-2014, 08:43 PM
#9
The 1.36 is what the boat came with, has been on there since I bought it in 99. A few years ago I swapped my 1.36 / 21 Mirage for a buddy's 1.5 / 23 Mirage Plus and saw virtually no difference in performance.
Maybe the dyno was a little "happy" - no real way to know for sure. The power numbers were in line with what Bob M expected with the cam and AFR heads that are being run - they weren't out of line for similar builds. Even if I'm not making the advertised power, it would have to be "happy" to the tune of at least 150 HP for these speeds.
Maybe the dyno was a little "happy" - no real way to know for sure. The power numbers were in line with what Bob M expected with the cam and AFR heads that are being run - they weren't out of line for similar builds. Even if I'm not making the advertised power, it would have to be "happy" to the tune of at least 150 HP for these speeds.
10-28-2014, 08:55 PM
#10
Registered
I ran my boat Sunday for what I was planning to be the final run for the season and finally had a chance to really crack on it before it went on the trailer. Seems like I always had a crapload of people on board every other time I had it out this year, so I went easy on it for most of the season. Top speed was considerably less than anticipated. I plugged in some numbers into the go-fast.com prop slip calculator, and I got the following slip calculations:
I am running a 1.36 Bravo 1 with a Mirage Plus 23. Boat is a 1988 Baja Force235 with a 489 running AFR heads, Holley 800, Lightnings, etc. This build has been pretty well documented on other threads here, so most of us are familiar with it. I had the engine dynoed and saw 585 HP at 5400 RPM and between 575-600 ft lbs of torque all the way from 3000 RPM up to peak HP.
At 3000 RPM I was seeing about 39 MPH on GPS. Theoretical MPH at this RPM is 49, so I have a slip factor of around 19% at this speed. Fair enough, there is a lot of hull in the water at this speed.
At 3500 RPM speed was showing about 48 MPH on the GPS. That equates to a slip factor of 14%, which is pretty much as expected.
I ran it on up to WOT, and was only able to make it to 61 MPH. RPM was only 4600, and that comes out to a slip factor of 18%. Something seems amiss here, both from the higher slip number and the fact that I wasn't able to spin this thing faster. I played around with trim and tabs, but did not seem to make a difference on top speed. Why would my slip number go up, unless I had it over trimmed?
It seems to be making plenty of power at lower speeds, getting up to 50 MPH is pretty effortless, so engine seems to be running OK. Did not note any miss or notice that it seemed to be running poorly, but it just wasn't bringing the power. Dyno runs were made with dry dyno headers, and A/F numbers were around 13 from 4000-5000 RPM, so I would expect it to run a little richer with the wet exhaust and muffler tips on it. I have yet to check the easy stuff like throttle opening all the way (it was with the other engine, and nothing changed in this regard), timing, etc. I was running speeds like this with a 454 that made less than 400 HP, so I would have to be dropping at least 100 HP if it is an issue with the engine.
Anyone want to take a stab at it? Maybe my boat is just a pig!
I am running a 1.36 Bravo 1 with a Mirage Plus 23. Boat is a 1988 Baja Force235 with a 489 running AFR heads, Holley 800, Lightnings, etc. This build has been pretty well documented on other threads here, so most of us are familiar with it. I had the engine dynoed and saw 585 HP at 5400 RPM and between 575-600 ft lbs of torque all the way from 3000 RPM up to peak HP.
At 3000 RPM I was seeing about 39 MPH on GPS. Theoretical MPH at this RPM is 49, so I have a slip factor of around 19% at this speed. Fair enough, there is a lot of hull in the water at this speed.
At 3500 RPM speed was showing about 48 MPH on the GPS. That equates to a slip factor of 14%, which is pretty much as expected.
I ran it on up to WOT, and was only able to make it to 61 MPH. RPM was only 4600, and that comes out to a slip factor of 18%. Something seems amiss here, both from the higher slip number and the fact that I wasn't able to spin this thing faster. I played around with trim and tabs, but did not seem to make a difference on top speed. Why would my slip number go up, unless I had it over trimmed?
It seems to be making plenty of power at lower speeds, getting up to 50 MPH is pretty effortless, so engine seems to be running OK. Did not note any miss or notice that it seemed to be running poorly, but it just wasn't bringing the power. Dyno runs were made with dry dyno headers, and A/F numbers were around 13 from 4000-5000 RPM, so I would expect it to run a little richer with the wet exhaust and muffler tips on it. I have yet to check the easy stuff like throttle opening all the way (it was with the other engine, and nothing changed in this regard), timing, etc. I was running speeds like this with a 454 that made less than 400 HP, so I would have to be dropping at least 100 HP if it is an issue with the engine.
Anyone want to take a stab at it? Maybe my boat is just a pig!
