Gludy
Active member
As the thread on this subject has almost dissappeared from the bottom of the page ( I wish forum admin would arrange thread ranking by latest post), I have started a new thread on this subject - the last thread ended with a number of points being raised about Jets that I have investigated and waish to answer in full:-
Drag caused through Jet intakes -
Intake velocity to a jet is almost exactly the same as the water moving along the hull so it is not right to say it travels very fast through a grill. There are of course losses associated with water friction and direction changing through a water jet but appendage drag is something else - it is a combination of friction and simple frontal area of appendages which resist movement through the water.
There are different types of surface drives some with rudders etc and so their drag will vary as well. A general rule of thumb is that water jets are most suited for boats with top speeds up to 45-50 knots. Above this speed the surface drive does show an improvement in efficiency over conventional jets. For jets to match theoretical efficiencies in this high range, special designs ie multi stage units or special intake configuration etc are necessary. We should also remember that to optimise any propulsor for high speed invariably causes some downside at other operating conditions.
In high speed craft, the engine position is a function of where they need to be to achieve optimum boat trim. It is in fact the surface drive that has the engines further aft because this is generally what is wanted for high speeds of course.
To compare true performance, we should first of all establish what tthe cutomer is looking for. Sheer top speeds ie in the 50 knots plus range, and the surface drives are very popular. This is a specialised section of the market and for the majority of owners, the huge list of water jet advantages far outweighs the ability to gain an extra one or two or even 5 knots!!
Yes, stopping in a hurry can result in getting seriously wet, but if the choice was get wet or hit an obstacle at 35 knots, I know what I would prefer ! Just because something CAN be done, does not mean that you have to do it every time!
The impeller clearance is not critical with the Hamilton design that I am researching. Units which use tapered impellers and bowls do indeed show drastic loss in performance and require frequent refurbishment if you operate at high power in very shallow and stoney conditions. Still what would happen to any sort of propeller if you abused it like that?
Corrosion is something which has to be considered on all craft and provided we stick to the manufacturers recommendations, there really should not be big problems.
So, from all the points that have been raised to date, for most pleasure boat applications it seems to me that not one point has been raised that favours props as the best means of propulsion. Props are old fashioned - I would like someone to really take me to pieces on this, as I can only learn more by them doing so.
So go on - let us know why props are better!
Paul
Drag caused through Jet intakes -
Intake velocity to a jet is almost exactly the same as the water moving along the hull so it is not right to say it travels very fast through a grill. There are of course losses associated with water friction and direction changing through a water jet but appendage drag is something else - it is a combination of friction and simple frontal area of appendages which resist movement through the water.
There are different types of surface drives some with rudders etc and so their drag will vary as well. A general rule of thumb is that water jets are most suited for boats with top speeds up to 45-50 knots. Above this speed the surface drive does show an improvement in efficiency over conventional jets. For jets to match theoretical efficiencies in this high range, special designs ie multi stage units or special intake configuration etc are necessary. We should also remember that to optimise any propulsor for high speed invariably causes some downside at other operating conditions.
In high speed craft, the engine position is a function of where they need to be to achieve optimum boat trim. It is in fact the surface drive that has the engines further aft because this is generally what is wanted for high speeds of course.
To compare true performance, we should first of all establish what tthe cutomer is looking for. Sheer top speeds ie in the 50 knots plus range, and the surface drives are very popular. This is a specialised section of the market and for the majority of owners, the huge list of water jet advantages far outweighs the ability to gain an extra one or two or even 5 knots!!
Yes, stopping in a hurry can result in getting seriously wet, but if the choice was get wet or hit an obstacle at 35 knots, I know what I would prefer ! Just because something CAN be done, does not mean that you have to do it every time!
The impeller clearance is not critical with the Hamilton design that I am researching. Units which use tapered impellers and bowls do indeed show drastic loss in performance and require frequent refurbishment if you operate at high power in very shallow and stoney conditions. Still what would happen to any sort of propeller if you abused it like that?
Corrosion is something which has to be considered on all craft and provided we stick to the manufacturers recommendations, there really should not be big problems.
So, from all the points that have been raised to date, for most pleasure boat applications it seems to me that not one point has been raised that favours props as the best means of propulsion. Props are old fashioned - I would like someone to really take me to pieces on this, as I can only learn more by them doing so.
So go on - let us know why props are better!
Paul
